Problem with my calendar. It shows those ads and I don’t know if it’s a virus or just ads.

It’s just ads. 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