The feature is available only on Server versions
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 customize 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". By default there are some fields, you can delete or add the fields you want. If you delete all fields, the default fields will be shown, so leave at least 1 field in the 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 Avatar and Full Name of Assignee, Reporter or Single User from Single user picker field in calendar events.
Updated events and legends will look like this:
