In the episode (in the series with Tory in)that Zack dates Slater's sister and the gang go to the "sock dance" at the Max, the poster on the school wall advertising the dance says "this Friday."

However Mr. Belding then complains to the boys that his wife is sending him on a male bonding course "this weekend" yet he returns from the course before the dance takes place...how can this be if Fridays come beofre the weekend?!