Spam on calendar

Check:

Unsubscribe from other Calendars 

To unsubscribe from a calendar other than an iCloud calendar: 

1. Go to Settings > Passwords & Accounts. 
2. Tap the account. 
3. Tap the slider to turn off Calendar. 
4. 
5. 
   
6. Tap Delete from My iPhone.

Please see: How to unsubscribe from calendars on your iPhone - Apple Support

Maybe I'm wrong, but the other solution just disabled calendar instead of fixing the reason.

This happened because you've subscribed to a third-party calendar somehow (most likely clicked on some spam link). To remove these, go to Settings > Accounts&passwords and delete all unknown/suspicious accounts that you see.

You are welcome.

This worked for me. Thank you