While its true the entire story of Wesley and Buttercup has nothing to do with Christmas, the framing device of the story...where Peter Falk reads the story to Fred Savage takes place during Christmas time. There is a Santa art project/decoration hanging on the closet wall behind Falk as well as a few other Christmas decorations throughout the room. Also, if you keep a sharp eye out at the very beginning, when Fred Savage's mother (sorry, actress, I don't know your name) opens the blinds, you can see the Christmas lights up and lit on the neighbor's house.
I've never been able to find out why Rob Reiner set this part at Christmas, but directors don't do this stuff on accident.
