From Online Manual
The calendar (en) feature, if enabled by the administrator, will be found on the Main menu (en) of the forum. It can be used to record and display holidays, birthdays, and events. Birthdays entered in members' user profiles will be 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 their 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, followed by Post Event from the calendar screen. If the option is enabled, users may also be able to simply select the day of the month to post a new event on that day. In addition to this, if a user has the proper permissions, they may see a Link to Calendar link 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 selected the Link to Calendar link.