Yeah unfortunately I agree this isn’t a bug. If you change seasons after you’ve played for a minute on a different setting it will completely mess up your holidays. Also I think some pre seasons saves can have this issue if you didn’t start on a certain day of the week but I’m not 100 percent on that. It does have a huge warning under the season length that changing it can cause holidays to get messed up too, so there really is no chance it’s anything but intentional. Only thing you can do is delete and remake then.