I grew up calling them possums, but spelling it opossum. Shrug.
I did have one of those mind blowing / reality altering situations a few years ago, though. I walked into a grocery store one day and it was lit up like Valentine's Day, but it was October. I thought to myself, "What the heck?" Did I miss something? I asked someone and they said it was Sweetest Day. Um, what? "Sweet-est DAY?" (Say this with a sneer, I did.) Sweetest day? Really? OOooooooooook. I had recently moved to the area at the time. I thought, much like Tyme machines and bubblers, that this was a Wisconsin thang.
So, I did what anyone would naturally do. I called my Mom. Mom is pretty much my litmus test for "should I have known about this?" Mom tells me, oh yeah, Sweetest day, been around for years.
(I still think they made it up a few years ago and the whole world is in on the conspiracy to try to convince me it actually is a holiday, regardless of how ridiculous it sounds, just to see if I'm gullible to believe it.)
Same here about the opossums. I did the same as you. As for sweetest day, never in my life have I heard of that...