The Calendar Edit History provides a history of all edits made to a Calendar by any Visitor to your Calendar with Edit Rights to the calendar. With the Calendar Edit History, you can protect calendar data with real-time backups and recover a deleted calendar entry.

As the owner of the calendar, you can access your Calendar Edit History by clicking on the gray "Customize" button in the blue bar above your calendar. 

However, Visitors that you have given Edit Rights to will not be able to see this button. In order to access your Calendar Edit History, your visitors with Edit Rights will need to click on an event in your calendar and then follow the steps in the image below: