From Online Manual
If the calendar is enabled by the Administrator it can be accessed via the Main Menu. The forum calendar is displayed exactly like any other calendar. Each day has its own box where events, birthdays, and holidays scheduled for that day are displayed.
If enabled, members can Post events to the calender, edit their events, and link events to topics.
The calendar also displays mini calendars on the left of the current calendar. This enables members to see what is coming up in the next month.