Winter is my favorite season, and Christmas is my favorite holiday, and so at this time of year I invariably find myself wanting to write wintry and Christmas-themed scenarios, even if my current WIP has nothing to do with either one.
I think it’s a valid writing exercise, though. Writing about a holiday that may or may not actually figure into your story is a great way to develop more details about your characters and world. Does your character hate Christmas and everything festive? Does your character celebrate Hanukkah but always wanted to put up a Christmas tree? How does the time period, nationality, and culture of your character affect their winter-time celebrations? Does your character live in a fantasy world with a holiday of your own creation?
Try writing a scene or a vignette about your main character(s) celebrating – or avoiding – their traditional winter holiday. It doesn’t have to fit into your current story – just write a scene and see where it goes! You might learn a lot about your characters and your story’s world!