Non-JIRA events can be added to the calendar which can be seen only in a particular calendar. Select one day or period you need and select "New event":

The event will be added to the current calendar and It has a limited number of parameters:

  1. Name. You can change it after the creation.
  2. Background and text color.
  3. 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 wrong one and create a new one.