Perhaps someone (or something), or even perhaps yourself, have added an additional Calendar to your device - and this unexpected Calendar is exposing unwanted calendar events and sending you unexpected reminders.
Let’s check for anything that’s out of place...
Settings > Calendar > Accounts
Look for an account that shouldn’t be in the list of accounts - as this will likely include the Calendar that contains all the unwanted events. When you find the suspect account, tap - then select Delete Account. This should resolve the problem in its entirety.
I hope you find this helpful in finding the source - and in resolving the issue if an unexpected Calendar is found