If you end up with pop-ups from your iPhone calendar, you most likely accidentally subscribed to a phony-baloney Malware-packed calendar. Just go to your list of “subscribed” calendars and remove it. Unfortunately, it can be hard to find as it’s un-named. I just helped a customer here in the shop with this exact issue for free. She was pleased I could take care of it in about a minute as she was on her way to the Apple Store in Annapolis for help.