A calendar is a schedule of layouts or playlists, based on date and time (if you’re looking to schedule based on duration rather than date/time, you should check playlists)

The calendar can be shown in Monthly, Weekly or Daily mode.
You can create and item at a specific date and time, and assign to it any of your layouts or playlists.
When you update a layout or playlist used by one or many calendars, the changes will automatically be reflected in the calendars within a few seconds.
Check this short animation showing how the calendar works: