From Online Manual
The calendar feature, if enabled by the administrator, is found in the Main menu 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 is enabled on a forum and users are given the permission to post calendar events, users will be able to Post an Event to the calendar by selecting Calendar from the main menu. Users can also simply click on a day in the calendar to post a new event on that day. In addition to this, users with the appropriate permissions may see Link to Calendar at the bottom of each topic. Selecting this link allows users to post a new event in the calendar, linked to the topic they were viewing when they click the Link to Calendar link.