[S] Can't plan goaled events on days with holidays

I have this problem too. I am trying to plan a custom wedding event, I don't do this very often, but I was super surprised that I can't plan it on the days that I want (the weekend). This is because there's already another event or activity.

From a cursory glance it seems to cause problems with:

• High School Events in particular, I can plan on Team Sports days.
• The Starlight Accolades on Sundays, when it appears on the calendar. If it isn't on the calendar yet, I can plan a wedding event.
• It works with the daily festivals of the new Life and Death pack. 
  It works with the festivals of City Living.

I don't know if it works with the Finchwick Fair of Cottage living, because it always coincides with High School activities/events on Saturday.

I don't use any mods at this time.