At first, the Calendar application should be added to your Jira. Read how to find, add and manage the Cloud apps (Atlassian documentation).
When the app is installed:
the user section becomes available (Apps → Calendar)
the Calendar gadget becomes available on dashboards
Calendar objects and concepts
⭐ The app allows users to create such objects as calendars and switch between them in the user section interface. A user can only browse one calendar at a time.
Each calendar displays a scope of issues and events – issues come from Jira projects while an event belongs to a particular calendar in which it was created.
Issues can be displayed as planned and unplanned. The latest ones are those complying with the calendar scope but having their timing unspecified.