From Online Manual
Calendar events, or just events, are user posted calendar entries.
Unlike holidays, these are usually tied to a topic for the purpose of discussion, but as of SMF 1.1 they can be independent of any topic. They can often be helpful in coordinating real-world events in many situations, or reminding people of upcoming happenings.
Events can only be posted, seen, and modified when the calendar is enabled. The number of events you can have posted is limited to about 65,535 by default, but this limit can be raised.