iCloud: Delete a calendar
When you delete a calendar from your Calendars list, all events associated with the calendar are also deleted.
You can’t delete all of the calendars in the list because it must always contain at least one calendar.
Click Edit below the sidebar.
Click the Delete button to the right of the calendar’s name, then click Delete.
If you accidentally deleted iCloud calendars or events, you can use iCloud.com to restore them, together with your reminders, from an earlier version that was automatically archived in iCloud. For instructions, see Restore your contacts, calendars and reminders, or bookmarks.