Any way to get rid of a calendar I did not subscribe to?

I just got a notification for Juneteenth. I didn't know what it is and I don't give a carp about it. The calendar it belongs to was turned off, yet the Event Alerts setting for it was still on. (It's weird that disabling a calendar doesn't also disable its alerts.) iOS doesn't show where a calendar came from, and this one has no Unsubscribe or Delete button. Another thread said it shows up automatically when you choose your calendar region (Gregorian, etc). I just want it gone, especially if it's going to show me bogus holidays.