diff --git a/src/Bundle/ChillCalendarBundle/Controller/InviteApiController.php b/src/Bundle/ChillCalendarBundle/Controller/InviteApiController.php
new file mode 100644
index 000000000..960459617
--- /dev/null
+++ b/src/Bundle/ChillCalendarBundle/Controller/InviteApiController.php
@@ -0,0 +1,69 @@
+<?php
+
+/**
+ * Chill is a software for social workers
+ *
+ * For the full copyright and license information, please view
+ * the LICENSE file that was distributed with this source code.
+ */
+
+declare(strict_types=1);
+
+namespace Chill\CalendarBundle\Controller;
+
+use Chill\CalendarBundle\Entity\Calendar;
+use Chill\CalendarBundle\Entity\Invite;
+use Chill\CalendarBundle\Security\Voter\InviteVoter;
+use Chill\MainBundle\Entity\User;
+use Doctrine\ORM\EntityManagerInterface;
+use Symfony\Component\HttpFoundation\JsonResponse;
+use Symfony\Component\HttpFoundation\Response;
+use Symfony\Component\HttpKernel\Exception\AccessDeniedHttpException;
+use Symfony\Component\HttpKernel\Exception\BadRequestHttpException;
+use Symfony\Component\Routing\Annotation\Route;
+use Symfony\Component\Security\Core\Security;
+use function in_array;
+
+class InviteApiController
+{
+    private EntityManagerInterface $entityManager;
+
+    private Security $security;
+
+    public function __construct(Security $security, EntityManagerInterface $entityManager)
+    {
+        $this->security = $security;
+        $this->entityManager = $entityManager;
+    }
+
+    /**
+     * Give an answer to a calendar invite.
+     *
+     * @Route("/api/1.0/calendar/calendar/{id}/answer/{answer}.json", methods={"post"})
+     */
+    public function answer(Calendar $calendar, string $answer): Response
+    {
+        $user = $this->security->getUser();
+
+        if (!$user instanceof User) {
+            throw new AccessDeniedHttpException('not a regular user');
+        }
+
+        if (null === $invite = $calendar->getInviteForUser($user)) {
+            throw new AccessDeniedHttpException('not invited to this calendar');
+        }
+
+        if (!$this->security->isGranted(InviteVoter::ANSWER, $invite)) {
+            throw new AccessDeniedHttpException('not allowed to answer on this invitation');
+        }
+
+        if (!in_array($answer, Invite::STATUSES, true) || Invite::PENDING === $answer) {
+            throw new BadRequestHttpException('answer not valid');
+        }
+
+        $invite->setStatus($answer);
+        $this->entityManager->flush();
+
+        return new JsonResponse(null, Response::HTTP_ACCEPTED, [], false);
+    }
+}
diff --git a/src/Bundle/ChillCalendarBundle/Entity/Calendar.php b/src/Bundle/ChillCalendarBundle/Entity/Calendar.php
index 2a05c1c77..af23c26c4 100644
--- a/src/Bundle/ChillCalendarBundle/Entity/Calendar.php
+++ b/src/Bundle/ChillCalendarBundle/Entity/Calendar.php
@@ -26,6 +26,7 @@ use DateInterval;
 use DateTimeImmutable;
 use Doctrine\Common\Collections\ArrayCollection;
 use Doctrine\Common\Collections\Collection;
+use Doctrine\Common\Collections\Criteria;
 use Doctrine\ORM\Mapping as ORM;
 use LogicException;
 use Symfony\Component\Serializer\Annotation as Serializer;
@@ -212,7 +213,7 @@ class Calendar implements TrackCreationInterface, TrackUpdateInterface
 
     public function addUser(User $user): self
     {
-        if (!$this->getUsers()->contains($user)) {
+        if (!$this->getUsers()->contains($user) && $this->getMainUser() !== $user) {
             $this->addInvite((new Invite())->setUser($user));
         }
 
@@ -263,6 +264,21 @@ class Calendar implements TrackCreationInterface, TrackUpdateInterface
         return $this->id;
     }
 
+    public function getInviteForUser(User $user): ?Invite
+    {
+        $criteria = Criteria::create();
+        $criteria->where(Criteria::expr()->eq('user', $user));
+
+        $matchings = $this->invites
+            ->matching($criteria);
+
+        if (1 === $matchings->count()) {
+            return $matchings->first();
+        }
+
+        return null;
+    }
+
     /**
      * @return Collection|Invite[]
      */
@@ -365,6 +381,18 @@ class Calendar implements TrackCreationInterface, TrackUpdateInterface
         return null !== $this->calendarRange;
     }
 
+    /**
+     * return true if the user is invited.
+     */
+    public function isInvited(User $user): bool
+    {
+        if ($this->getMainUser() === $user) {
+            return false;
+        }
+
+        return $this->getUsers()->contains($user);
+    }
+
     public static function loadValidatorMetadata(ClassMetadata $metadata): void
     {
         $metadata->addPropertyConstraint('startDate', new NotBlank());
@@ -485,6 +513,7 @@ class Calendar implements TrackCreationInterface, TrackUpdateInterface
         }
 
         $this->mainUser = $mainUser;
+        $this->removeUser($mainUser);
 
         return $this;
     }
diff --git a/src/Bundle/ChillCalendarBundle/Entity/Invite.php b/src/Bundle/ChillCalendarBundle/Entity/Invite.php
index 751727af8..be80a3e56 100644
--- a/src/Bundle/ChillCalendarBundle/Entity/Invite.php
+++ b/src/Bundle/ChillCalendarBundle/Entity/Invite.php
@@ -36,6 +36,16 @@ class Invite implements TrackUpdateInterface, TrackCreationInterface
 
     public const PENDING = 'pending';
 
+    /**
+     * all statuses in one const.
+     */
+    public const STATUSES = [
+        self::ACCEPTED,
+        self::DECLINED,
+        self::PENDING,
+        self::TENTATIVELY_ACCEPTED,
+    ];
+
     public const TENTATIVELY_ACCEPTED = 'tentative';
 
     /**
diff --git a/src/Bundle/ChillCalendarBundle/RemoteCalendar/Connector/MSGraph/RemoteEventConverter.php b/src/Bundle/ChillCalendarBundle/RemoteCalendar/Connector/MSGraph/RemoteEventConverter.php
index f73fcd31b..fcc74cb1c 100644
--- a/src/Bundle/ChillCalendarBundle/RemoteCalendar/Connector/MSGraph/RemoteEventConverter.php
+++ b/src/Bundle/ChillCalendarBundle/RemoteCalendar/Connector/MSGraph/RemoteEventConverter.php
@@ -21,7 +21,6 @@ use DateTimeImmutable;
 use DateTimeZone;
 use Symfony\Component\Templating\EngineInterface;
 use Symfony\Contracts\Translation\TranslatorInterface;
-use Twig\Environment;
 
 /**
  * Convert Chill Calendar event to Remote MS Graph event, and MS Graph
@@ -35,14 +34,14 @@ class RemoteEventConverter
 
     private DateTimeZone $defaultDateTimeZone;
 
+    private EngineInterface $engine;
+
     private PersonRenderInterface $personRender;
 
     private DateTimeZone $remoteDateTimeZone;
 
     private TranslatorInterface $translator;
 
-    private EngineInterface $engine;
-
     public function __construct(EngineInterface $engine, PersonRenderInterface $personRender, TranslatorInterface $translator)
     {
         $this->engine = $engine;
@@ -112,7 +111,7 @@ class RemoteEventConverter
                         '@ChillCalendar/MSGraph/calendar_event_body.html.twig',
                         ['calendar' => $calendar]
                     ),
-                ]
+                ],
             ],
             $this->calendarToEventAttendeesOnly($calendar)
         );
@@ -129,17 +128,6 @@ class RemoteEventConverter
         ];
     }
 
-    private function buildInviteToAttendee(Invite $invite): array
-    {
-        return [
-            'emailAddress' => [
-                'address' => $invite->getUser()->getEmail(),
-                'name' => $invite->getUser()->getLabel(),
-            ],
-            'type' => 'Required',
-        ];
-    }
-
     public function convertAvailabilityToRemoteEvent(array $event): RemoteEvent
     {
         $startDate =
@@ -193,4 +181,15 @@ class RemoteEventConverter
     {
         return new DateTimeZone('UTC');
     }
+
+    private function buildInviteToAttendee(Invite $invite): array
+    {
+        return [
+            'emailAddress' => [
+                'address' => $invite->getUser()->getEmail(),
+                'name' => $invite->getUser()->getLabel(),
+            ],
+            'type' => 'Required',
+        ];
+    }
 }
diff --git a/src/Bundle/ChillCalendarBundle/RemoteCalendar/Connector/MSGraphRemoteCalendarConnector.php b/src/Bundle/ChillCalendarBundle/RemoteCalendar/Connector/MSGraphRemoteCalendarConnector.php
index 80a5dd792..5def925c2 100644
--- a/src/Bundle/ChillCalendarBundle/RemoteCalendar/Connector/MSGraphRemoteCalendarConnector.php
+++ b/src/Bundle/ChillCalendarBundle/RemoteCalendar/Connector/MSGraphRemoteCalendarConnector.php
@@ -28,6 +28,7 @@ use Symfony\Component\HttpFoundation\Response;
 use Symfony\Component\Routing\Generator\UrlGeneratorInterface;
 use Symfony\Contracts\HttpClient\Exception\ClientExceptionInterface;
 use Symfony\Contracts\Translation\TranslatorInterface;
+use function count;
 
 class MSGraphRemoteCalendarConnector implements RemoteCalendarConnectorInterface
 {
@@ -41,10 +42,10 @@ class MSGraphRemoteCalendarConnector implements RemoteCalendarConnectorInterface
 
     private RemoteEventConverter $remoteEventConverter;
 
-    private TranslatorInterface $translator;
-
     private OnBehalfOfUserTokenStorage $tokenStorage;
 
+    private TranslatorInterface $translator;
+
     private UrlGeneratorInterface $urlGenerator;
 
     private OnBehalfOfUserHttpClient $userHttpClient;
@@ -171,7 +172,7 @@ class MSGraphRemoteCalendarConnector implements RemoteCalendarConnectorInterface
                     $calendar->getRemoteId(),
                     $this->translator->trans('remote_ms_graph.cancel_event_because_main_user_is_%label%', ['%label%' => $calendar->getMainUser()]),
                     $previousMainUser,
-                    'calendar_'.$calendar->getRemoteId()
+                    'calendar_' . $calendar->getRemoteId()
                 );
                 $this->createCalendarOnRemote($calendar);
             } else {
@@ -189,10 +190,9 @@ class MSGraphRemoteCalendarConnector implements RemoteCalendarConnectorInterface
                 ->addRemoteAttributes([
                     'lastModifiedDateTime' => null,
                     'changeKey' => null,
-                    'previousId' => $calendar->getCalendarRange()->getRemoteId()
+                    'previousId' => $calendar->getCalendarRange()->getRemoteId(),
                 ])
-                ->setRemoteId('')
-                ;
+                ->setRemoteId('');
         }
 
         if (null !== $previousCalendarRange) {
@@ -209,31 +209,30 @@ class MSGraphRemoteCalendarConnector implements RemoteCalendarConnectorInterface
         }
     }
 
-    private function patchCalendarOnRemote(Calendar $calendar, array $newInvites): void
+    private function cancelOnRemote(string $remoteId, string $comment, User $user, string $identifier): void
     {
-        $eventDatas = [];
-        $eventDatas[] = $this->remoteEventConverter->calendarToEvent($calendar);
+        $userId = $this->mapCalendarToUser->getUserId($user);
 
-        if (0 < count($newInvites)) {
-            $eventDatas[] = $this->remoteEventConverter->calendarToEventAttendeesOnly($calendar);
+        if (null === $userId) {
+            return;
         }
 
-        foreach ($eventDatas as $eventData) {
-            [
-                'id' => $id,
-                'lastModifiedDateTime' => $lastModified,
-                'changeKey' => $changeKey
-            ] = $this->patchOnRemote(
-                $calendar->getRemoteId(),
-                $eventData,
-                $calendar->getMainUser(),
-                'calendar_'.$calendar->getId()
+        try {
+            $this->machineHttpClient->request(
+                'POST',
+                "users/{$userId}/calendar/events/{$remoteId}/cancel",
+                [
+                    'json' => ['Comment' => $comment],
+                ]
             );
-
-            $calendar->addRemoteAttributes([
-                'lastModifiedDateTime' => $lastModified,
-                'changeKey' => $changeKey,
+        } catch (ClientExceptionInterface $e) {
+            $this->logger->warning('could not update calendar range to remote', [
+                'exception' => $e->getTraceAsString(),
+                'content' => $e->getResponse()->getContent(),
+                'calendarRangeId' => $identifier,
             ]);
+
+            throw $e;
         }
     }
 
@@ -330,7 +329,7 @@ class MSGraphRemoteCalendarConnector implements RemoteCalendarConnectorInterface
         $calendarRange->setRemoteId($id)
             ->addRemoteAttributes([
                 'lastModifiedDateTime' => $lastModified,
-                'changeKey' => $changeKey
+                'changeKey' => $changeKey,
             ]);
     }
 
@@ -404,33 +403,32 @@ class MSGraphRemoteCalendarConnector implements RemoteCalendarConnectorInterface
         );
     }
 
-    private function cancelOnRemote(string $remoteId, string $comment, User $user, string $identifier): void
+    private function patchCalendarOnRemote(Calendar $calendar, array $newInvites): void
     {
-        $userId = $this->mapCalendarToUser->getUserId($user);
+        $eventDatas = [];
+        $eventDatas[] = $this->remoteEventConverter->calendarToEvent($calendar);
 
-        if (null === $userId) {
-            return;
+        if (0 < count($newInvites)) {
+            $eventDatas[] = $this->remoteEventConverter->calendarToEventAttendeesOnly($calendar);
         }
 
-        try {
-            $this->machineHttpClient->request(
-                'POST',
-                "users/{$userId}/calendar/events/{$remoteId}/cancel",
-                [
-                    'json' => ['Comment' => $comment]
-                ]
+        foreach ($eventDatas as $eventData) {
+            [
+                'id' => $id,
+                'lastModifiedDateTime' => $lastModified,
+                'changeKey' => $changeKey
+            ] = $this->patchOnRemote(
+                $calendar->getRemoteId(),
+                $eventData,
+                $calendar->getMainUser(),
+                'calendar_' . $calendar->getId()
             );
-        } catch (ClientExceptionInterface $e) {
-            $this->logger->warning('could not update calendar range to remote', [
-                'exception' => $e->getTraceAsString(),
-                'content' => $e->getResponse()->getContent(),
-                'calendarRangeId' => $identifier,
+
+            $calendar->addRemoteAttributes([
+                'lastModifiedDateTime' => $lastModified,
+                'changeKey' => $changeKey,
             ]);
-
-            throw $e;
         }
-
-
     }
 
     /**
diff --git a/src/Bundle/ChillCalendarBundle/Security/Voter/InviteVoter.php b/src/Bundle/ChillCalendarBundle/Security/Voter/InviteVoter.php
new file mode 100644
index 000000000..83caa5f64
--- /dev/null
+++ b/src/Bundle/ChillCalendarBundle/Security/Voter/InviteVoter.php
@@ -0,0 +1,35 @@
+<?php
+
+/**
+ * Chill is a software for social workers
+ *
+ * For the full copyright and license information, please view
+ * the LICENSE file that was distributed with this source code.
+ */
+
+declare(strict_types=1);
+
+namespace Chill\CalendarBundle\Security\Voter;
+
+use Chill\CalendarBundle\Entity\Invite;
+use Symfony\Component\Security\Core\Authentication\Token\TokenInterface;
+use Symfony\Component\Security\Core\Authorization\Voter\Voter;
+
+class InviteVoter extends Voter
+{
+    public const ANSWER = 'CHILL_CALENDAR_INVITE_ANSWER';
+
+    protected function supports($attribute, $subject): bool
+    {
+        return $subject instanceof Invite && self::ANSWER === $attribute;
+    }
+
+    /**
+     * @param string $attribute
+     * @param Invite $subject
+     */
+    protected function voteOnAttribute($attribute, $subject, TokenInterface $token): bool
+    {
+        return $token->getUser() === $subject->getUser();
+    }
+}
diff --git a/src/Bundle/ChillCalendarBundle/chill.api.specs.yaml b/src/Bundle/ChillCalendarBundle/chill.api.specs.yaml
index 98e90d20d..1c055a24e 100644
--- a/src/Bundle/ChillCalendarBundle/chill.api.specs.yaml
+++ b/src/Bundle/ChillCalendarBundle/chill.api.specs.yaml
@@ -1,204 +1,240 @@
 ---
-openapi: "3.0.0"
-info:
-  version: "1.0.0"
-  title: "Chill api"
-  description: "Api documentation for chill. Currently, work in progress"
-servers:
-  - url: "/api"
-    description: "Your current dev server"
+#openapi: "3.0.0"
+#info:
+#  version: "1.0.0"
+#  title: "Chill api"
+#  description: "Api documentation for chill. Currently, work in progress"
+#servers:
+#  - url: "/api"
+#    description: "Your current dev server"
 
 components:
-  schemas:
-    Date:
-      type: object
-      properties:
-        datetime:
-          type: string
-          format: date-time
-    User:
-      type: object
-      properties:
-        id:
-          type: integer
-        type:
-          type: string
-          enum:
-            - user
-        username:
-          type: string
-        text:
-          type: string
+    schemas:
+        Date:
+            type: object
+            properties:
+                datetime:
+                    type: string
+                    format: date-time
+        User:
+            type: object
+            properties:
+                id:
+                    type: integer
+                type:
+                    type: string
+                    enum:
+                        - user
+                username:
+                    type: string
+                text:
+                    type: string
 
 paths:
-  /1.0/calendar/calendar.json:
-    get:
-      tags:
-        - calendar
-      summary: Return a list of all calendar items
-      responses:
-        200:
-          description: "ok"
+    /1.0/calendar/calendar/{id}/answer/{answer}.json:
+        post:
+            tags:
+                - calendar
+            summary: Answer to a calendar's invite
+            parameters:
+                -
+                    in: path
+                    name: id
+                    required: true
+                    description: the calendar id
+                    schema:
+                        type: integer
+                        format: integer
+                        minimum: 0
+                -
+                    in: path
+                    name: answer
+                    required: true
+                    description: the answer
+                    schema:
+                        type: string
+                        enum:
+                            - accepted
+                            - declined
+                            - tentative
+            responses:
+                400:
+                    description: bad answer
+                403:
+                    description: not invited
+                404:
+                    description: not found
+                202:
+                    description: accepted
 
-  /1.0/calendar/calendar/{id}.json:
-    get:
-      tags:
-        - calendar
-      summary: Return an calendar item by id
-      parameters:
-        - name: id
-          in: path
-          required: true
-          description: The calendar id
-          schema:
-            type: integer
-            format: integer
-            minimum: 1
-      responses:
-        200:
-          description: "ok"
-        404:
-          description: "not found"
-        401:
-          description: "Unauthorized"
+    /1.0/calendar/calendar.json:
+        get:
+            tags:
+                - calendar
+            summary: Return a list of all calendar items
+            responses:
+                200:
+                    description: "ok"
 
-  /1.0/calendar/calendar-range.json:
-    get:
-      tags:
-        - calendar
-      summary: Return a list of all calendar range items
-      responses:
-        200:
-          description: "ok"
-    post:
-      tags:
-        - calendar
-      summary: create a new calendar range
-      requestBody:
-        required: true
-        content:
-          application/json:
-            schema:
-              type: object
-              properties:
-                user:
-                  $ref: '#/components/schemas/User'
-                startDate:
-                  $ref: '#/components/schemas/Date'
-                endDate:
-                  $ref: '#/components/schemas/Date'
-      responses:
-        401:
-          description: "Unauthorized"
-        404:
-          description: "Not found"
-        200:
-          description: "OK"
-        422:
-          description: "Unprocessable entity (validation errors)"
-        400:
-          description: "transition cannot be applyed"
+    /1.0/calendar/calendar/{id}.json:
+        get:
+            tags:
+                - calendar
+            summary: Return an calendar item by id
+            parameters:
+                -   name: id
+                    in: path
+                    required: true
+                    description: The calendar id
+                    schema:
+                        type: integer
+                        format: integer
+                        minimum: 1
+            responses:
+                200:
+                    description: "ok"
+                404:
+                    description: "not found"
+                401:
+                    description: "Unauthorized"
 
-  /1.0/calendar/calendar-range/{id}.json:
-    get:
-      tags:
-        - calendar
-      summary: Return an calendar-range item by id
-      parameters:
-        - name: id
-          in: path
-          required: true
-          description: The calendar-range id
-          schema:
-            type: integer
-            format: integer
-            minimum: 1
-      responses:
-        200:
-          description: "ok"
-        404:
-          description: "not found"
-        401:
-          description: "Unauthorized"
-    patch:
-      tags:
-        - calendar
-      summary: update a calendar range
-      requestBody:
-        required: true
-        content:
-          application/json:
-            schema:
-              type: object
-              properties:
-                user:
-                  $ref: '#/components/schemas/User'
-                startDate:
-                  $ref: '#/components/schemas/Date'
-                endDate:
-                  $ref: '#/components/schemas/Date'
-      responses:
-        401:
-          description: "Unauthorized"
-        404:
-          description: "Not found"
-        200:
-          description: "OK"
-        422:
-          description: "Unprocessable entity (validation errors)"
-        400:
-          description: "transition cannot be applyed"
-    delete:
-      tags:
-        - calendar
-      summary: "Remove a calendar range"
-      parameters:
-        - name: id
-          in: path
-          required: true
-          description: The calendar range id
-          schema:
-            type: integer
-            format: integer
-            minimum: 1
-      responses:
-        401:
-          description: "Unauthorized"
-        404:
-          description: "Not found"
-        200:
-          description: "OK"
-        422:
-          description: "object with validation errors"
+    /1.0/calendar/calendar-range.json:
+        get:
+            tags:
+                - calendar
+            summary: Return a list of all calendar range items
+            responses:
+                200:
+                    description: "ok"
+        post:
+            tags:
+                - calendar
+            summary: create a new calendar range
+            requestBody:
+                required: true
+                content:
+                    application/json:
+                        schema:
+                            type: object
+                            properties:
+                                user:
+                                    $ref: '#/components/schemas/User'
+                                startDate:
+                                    $ref: '#/components/schemas/Date'
+                                endDate:
+                                    $ref: '#/components/schemas/Date'
+            responses:
+                401:
+                    description: "Unauthorized"
+                404:
+                    description: "Not found"
+                200:
+                    description: "OK"
+                422:
+                    description: "Unprocessable entity (validation errors)"
+                400:
+                    description: "transition cannot be applyed"
 
-  /1.0/calendar/calendar-range-available/{userId}.json:
-    get:
-      tags:
-        - calendar
-      summary: Return a list of available calendar range items. Available means calendar-range not being taken by a calendar entity
-      parameters:
-          - name: userId
-            in: path
-            required: true
-            description: The user id
-            schema:
-                type: integer
-                format: integer
-                minimum: 1
-          - name: dateFrom
-            in: query
-            required: true
-            description: The date from, formatted as ISO8601 string
-            schema:
-                type: string
-                format: date-time
-          - name: dateTo
-            in: query
-            required: true
-            description: The date to, formatted as ISO8601 string
-            schema:
-                type: string
-                format: date-time
-      responses:
-        200:
-          description: "ok"
+    /1.0/calendar/calendar-range/{id}.json:
+        get:
+            tags:
+                - calendar
+            summary: Return an calendar-range item by id
+            parameters:
+                -   name: id
+                    in: path
+                    required: true
+                    description: The calendar-range id
+                    schema:
+                        type: integer
+                        format: integer
+                        minimum: 1
+            responses:
+                200:
+                    description: "ok"
+                404:
+                    description: "not found"
+                401:
+                    description: "Unauthorized"
+        patch:
+            tags:
+                - calendar
+            summary: update a calendar range
+            requestBody:
+                required: true
+                content:
+                    application/json:
+                        schema:
+                            type: object
+                            properties:
+                                user:
+                                    $ref: '#/components/schemas/User'
+                                startDate:
+                                    $ref: '#/components/schemas/Date'
+                                endDate:
+                                    $ref: '#/components/schemas/Date'
+            responses:
+                401:
+                    description: "Unauthorized"
+                404:
+                    description: "Not found"
+                200:
+                    description: "OK"
+                422:
+                    description: "Unprocessable entity (validation errors)"
+                400:
+                    description: "transition cannot be applyed"
+        delete:
+            tags:
+                - calendar
+            summary: "Remove a calendar range"
+            parameters:
+                -   name: id
+                    in: path
+                    required: true
+                    description: The calendar range id
+                    schema:
+                        type: integer
+                        format: integer
+                        minimum: 1
+            responses:
+                401:
+                    description: "Unauthorized"
+                404:
+                    description: "Not found"
+                200:
+                    description: "OK"
+                422:
+                    description: "object with validation errors"
+
+    /1.0/calendar/calendar-range-available/{userId}.json:
+        get:
+            tags:
+                - calendar
+            summary: Return a list of available calendar range items. Available means calendar-range not being taken by a calendar entity
+            parameters:
+                -   name: userId
+                    in: path
+                    required: true
+                    description: The user id
+                    schema:
+                        type: integer
+                        format: integer
+                        minimum: 1
+                -   name: dateFrom
+                    in: query
+                    required: true
+                    description: The date from, formatted as ISO8601 string
+                    schema:
+                        type: string
+                        format: date-time
+                -   name: dateTo
+                    in: query
+                    required: true
+                    description: The date to, formatted as ISO8601 string
+                    schema:
+                        type: string
+                        format: date-time
+            responses:
+                200:
+                    description: "ok"