Hi, just to let you know I managed to correct this bug by downloading MCCC and changing the season. I don't know what caused, but I managed to correct it anyway. Thank you so much
I had the same problem today, but it was the first sims year I was playing in that save. I started the game in spring and everything was normal, then the last day of autumn I celebrated a wedding in my active household in a Brindleton Bay community lot and stayed in there till past midnight. Then I ended the event earlier and returned to the home lot in Windenburg and that's when I noticed that the bottom UI and the calendar were showing the first day of autumn again, despite the weather and the holidays were the ones of winter.
I just had the default holidays in the calendar (and the random ones that can pop up), the sims that got married were already part of the household from the start and I remember the last holiday they celebrated before the wedding has been Harvestfest.