Any user who has edit calendar permission can set up a custom fields in calendar events and their legends. From the JIRA main menu go to the "Calendar" and on the right side click calendar's Cog then chose "Set Up" menu:
Select Event layout menu.
To add custom fields in calendar events legend simply select from dropdown list or start to type field's name in the "Show custom fields in calendar event legend".
You can also add fields to the calendar event. Select a custom fields in the "Show custom field in calendar events". No more than tree fields can be added.
Also you can enable or disable the Assignee Avatar and Assignee Full Name in calendar events.
Updated events and legends will look like this: