Find a Pikachu in Pokemon GO anytime before December 29th and there's a chance it will be wearing a Santa hat, to celebrate Christmas.
Or "non-denominational winter-centric festival", as the PC squad probably want us to call it!!!
Niantic has dramatically increased the number of Pikachus stonin' about, so you should have a good chance to grab one. And, yes, the hat will still be there if you evolve Pikachu into Raichu.
So I guess our guide is: go outside. Yes. We're the best.