From Online Manual
The calendar (en) feature, if enabled by the administrator, is found in the Main menu (en) of the forum. It can be used to record and display holidays, birthdays, and events. Birthdays entered in members' profiles are displayed on the calendar. Some or all members may have permission to post calendar events. Administrators can configure the calendar via the calendar settings section of the Administration Center.
Posting an event to the calendar
If the calendar feature is enabled and users are given the required permission, events can be posted to the calendar by clicking on Post Event below the main calendar view.
Users with the appropriate permissions may see Link to Calendar at the bottom of each topic. This button enables users to create a new calendar event that is linked to the currently viewed topic.