Hi @FairClarity ! Definitely won't ask you to test without mods 😉 . Did you switch the length of seasons at some point after starting to play with that pack? That's the only cause I've seen for this. There are two fixes I've seen: One is to switch the season length to 7 days and back again (or similarly, change the season length and then back to where you want it). The other is to remake the holidays.
I decided to just make the seasons 14 days long because why not, and it fixed it. Just going from 7 to 14 days fixed it. It does still say Winterfest is today, but it actually has Winterfest in Winter too (Aka, Winterfest is on the calendar twice). I just had my sims drink a Moodlet Solver to get rid of the bad effect of ending the holiday.
I don't remember ever messing with how long the seasons are at any point. I might have, it's been a while since I've messed with this save, so I've got no idea. But it looks like changing the length of the season fixed it.
I suspect that 7 is really tricky to manage without deleting and recreating holidays, unfortunately.
