View all Calendars in your tenant
Just got this answer from Atlassian Support that I thought was so good it was worth reposting here for anyone interested.
You can search Calendars on your instance by Accessing the Advanced Search functionality on your instance.
This will take you to a page similar to this next screenshot. Under the field Type Select Calendars and you should be able to list all calendars on your instance (that are not restricted).
