Virus on my iPhone XR

You do not have a virus. This is what we call a rogue calendar. Don't worry, all your data is safe. Open up your Calendar app. Tap on Calendars at the bottom of the screen, then tap Edit at the top. Find the calendar that's making these "alerts" in the list, then tap the letter I symbol next to its name. Then, scroll down and tap Delete Calendar, and press Delete Calendar again on the popup at the bottom of your screen. After doing this I suggest that you change your Apple ID password ASAP to prevent this from happening again.

first of all delte cookie and internet data, close the browser, delete the things that you have in calendar , if you have some of important download from App Store a similar calendar application .or delete your datas by iPhone

Here is the right solution: Settings/Passwords & Accounts. Scroll to Subscribed Calendars, and delete the rogue calendar.

BTW, it got there because a website offered it to you, you click on a link, and iOS posted a dialog asking you to accept it.

Check Settings —> General —> Profiles. Anything there that you can delete?

The solution offered by Lawrence Finch was perfect and solved the problem in ten seconds. Thank You Mr. Finch !!!