Why do I have a virus notification in my calendar

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. 

The Apple support article is below. 

https://support.apple.com/en-us/HT211076

And to answer your "why?" question with a side trip into "how?", some website offered a "do you want notifications?" webcal link, or maybe some mail message arrived and offered a webcal subscription (haven't tried that path), and the calendar subscription offer was accepted, and the calendar was subscribed.

As happens with many of these cases, the calendar is then filled with advertisements.