Erase a notification from the calendar

That is a Rogue Calendar account. Go to Settings-->>Passwords & Accounts and under Accounts see if an unknown calendar account is listed. If it is then tap on it and then Delete account.

It is something that got added to your calendar because you may have accepted an offer from a pop-up  on a website unknowingly. Please be careful out there,

Good Luck.

Axel F

The answer depends a lot on the site and the nature of the notifications.

If you downloaded a calendar event (.ics file) from the site, then the events should appear on your calendar and you can select them and delete them from there. If it's a repeating event then Calendar.app will ask you if you want to delete the selected event or all future events.

It's also possible that you subscribed to a calendar, in which case it will appear in the sidebar in your Calendar window - just uncheck it to hide the items from your calendar, or Ctrl-click the calendar and select 'Unsubscribe' to disconnect and delete all the calendar items.

If neither of those work then you'll need to be more specific about the nature of these events.