If you're event isn't happening anymore, no biggie.. just cancel it.
The attendees are notified and the post remains in their feed, but has "cancelled" in it. Any reminders that were set will be removed.
1. Click on the post you wish to cancel.
2. Click “cancel”.
3. Show that cancellation who's boss .
4. Btw, if you made a mistake (“who, me? never!) you can undo that cancellation immediately in the upper right hand corner.