I hate to break it to you, but this is far more mundane than “viruses” or “hackers”.
You were offered a calendar from some website, and decided to subscribe to it for events and notifications, and you’re now getting the events from that subscribed calendar. Spam advertising, in this case.
As you no longer wish to subscribe to thIs calendar, delete the subscription.
Tap on the following blue-text link for instructions how to delete the unwanted subscribed calendar: Delete spam calendars and events on iPhone - Apple Support
Also of interest will be some of the other ways we get fooled by the advertisers and the scammers > Recognize and avoid phishing messages, phony support calls, and other scams - Apple Support