Along with issues a calendar can contain other kinds of entities.
Events
An event exists only in a calendar it was created. It can be added to a calendar by any calendar user in a similar way as creating a new issue.
An event has a limited number of parameters:
Name. You can change it after the creation.
Background and text color.
Schedule.
Once you've created an event you can change its name, color, and end date, however, it is impossible to reschedule already created events. You have to delete the incorrect one and create a new one.