Question marked as
You got tricked into installing a rogue calendar; your phone does not have a virus.
If your phone has iOS 13 or earlier: Go to Settings/Passwords & Accounts, look under Subscribed Calendars, tap on the offending calendar and delete it.
If your phone has iOS 14: Go to Settings/Calendar/Accounts, look under Subscribed Calendars, tap on the offending calendar and delete it.
And in the future beware of "free" offers