I'm losing my mind... how do I remove these old holidays?!

At some point in the last 15 years, I added a category of holidays to my Outlook calendar. They automatically appear every year. I cannot find the source of these holidays and I want them gone! I checked my settings and the "add holidays" option. Nothing. I checked my subscribed calendars. Nothing. Each holiday is an individual event so I can't delete a group at once. There's nothing to turn off or unsubscribe from. If I delete each individual holiday for the year, they still appear next year. I use New Outlook, but have switched back to the Old Outlook to try to find the source. Nothing. I suspect I added a holiday list years ago and somehow the list doesn't exist anymore but the holidays remain. I'M LOSING MY MIND! Can anyone help? Any advice??