Calendar alert virus

It's not a virus. you seem to have added a spam/scam calendar

Take a look at this article

Recognize and avoid phishing messages, phony support calls, and other scams - Apple Support

If you unintentionally subscribed to a calendar you didn't want, open the Calendar app, tap Calendars and look for an unknown or suspicious calendar. Tap the calendar, then scroll down and tap Delete Calendar.

Good luck!

Ignore this spam Calendar account.

In Calendar app, tap on Calendars and uncheck the ones you don’t recognise .

Then in:-

iOS13 Go to Settings, Passwords & Accounts and look in Accounts 

iOS14 Go to Settings, Calendar and look in Accounts 

And in either case see if a strange calendar is listed. If it is then tap on it and tap Delete account.