Calendars are a structure that host events, tasks and budgets.
Before you can start to work with events, tasks and budgets, you need to create at least one calendar. In a real world, you will need more than one - to provide a copy flow and calendar workstates.
Calendars are similar to folders, but they're a separate structure.
A typical calendar structure may look like:
As you notice, there could be many calendars, and every calendar may have sub-calendars.
Calendar structure is of arbitrary length and depth. You can base it on desks (News, Features, Central), on publications and workstates (Daily Echo) etc.
Besides hosting, calendars provide a calendar copy flow, e.g. distinguishing tasks by their workstate (open, closed etc.). Tasks do not have their own workstates, but inherit a workstate from the calendar where they reside. That makes calendars similar to folders.
See also