Userscript to delete all your past events on Facebook.
- Firefox: install Greasemonkey. Chrome: install Tampermonkey
- Install the user script from https://openuserjs.org/scripts/klausi/Facebook_delete_past_events
- Go to https://www.facebook.com/events/calendar/past/
- Click the "Delete all past events" button. If the button does not show up reload the page with F5.
- Wait until all events are deleted.
- If the browser crashes just close it and start the script again at https://www.facebook.com/events/calendar/past/