[SEASONS] Calender working weirdly

When I try to set an event (a birthday) on Sunday the calendar for some reason places it two mondays later and I don't know why. If I try to set in on that Saturday it simply doesn't appear on the calendar

https://i.imgur.com/VYByPCp.png

I tried to use it without any mod and no luck, I'm going to try and repair the game but I'm pretty sure it won't work anyway. Any idea why this happens?