It is highly probable that you have been manipulated (via a simple click on a website link) into “subscribing” an additional (unwanted) Calendar to your device - and this unexpected Calendar is exposing unwanted calendar events and/or sending you unexpected “adverts” or other warnings.
If you see this issue, you’ll need to check for what’s out of place...
- iOS/iPadOS13 and earlier: Settings > Passwords and Accounts
- iOS/iPadOS14/15/1617: Settings > Calendar > Accounts
- iOS/iPadOS18: Settings > Calendar > 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/if you find the suspect account, tap - then select Delete Account. This should resolve this specific problem in its entirety.