Hack alert

That’s a calendar you’ve subscribed to, and the folks that offered that calendar to you have filled it with spam.

Here is how to Remove spam from Calendar on your iPhone - Apple Support

If running iOS 13 or earlier versions, check: Settings - Passwords & Accounts - Accounts - Any rogue entries here? If so, delete the rogue account. 

If running iOS 14, check: Settings - Calendar - Accounts - Any rogue entries here? If so, delete the rogue account. 

De nada.

Here are some of other sorts of scams that are around:

Scam Pop-ups, Scam Viruses, Scam Receipts… - Apple Community