Removing “interested in” Facebook events from calendar

I clicked that I was interested in a 6 week long event on Facebook. I later changed my response to not going, however the event is still in my calendar on my phone. So annoying, as it looks as though I have something on my calendar every day thru December. I have turned FB events off and back on in my calendar, along with the ‘show declined FB events’ button but nothing is working. My iPhone calendar settings match my iPad and don’t have this issue with the iPad. I have backed up and synced hoping that may help.

Does anybody know how to fix this?