There are several sections where you find calendars:
Global Calendar Section
Project Calendar Section
Issue Calendar Section
Calendar for Jira Service Desk Customers
They have the same concept of displaying and planning issues but slightly different content, functionality and settings.
There are global permissions that define which user groups can use the Calendar functionality. If you cannot access Calendar contact your Jira administrator.
Read more: Calendar Global Permissions
Along with putting issues from the Unplanned section to the calendar grid, you can use other planning methods such as:
add a new issue directly from the calendar
put the existing issue on a particular calendar place via search
drag-and-drop an issue or stretch the issue timeframes within the calendar
drop the dates by removing an issue from the calendar
Planning options may differ according to a particular section using as well as the user project permissions.
Read more: Planning