Get into the festive shit. I never cared about Chirstmas or anything, so December was just cold/dark time. Wife was kind of in the same boat, but one year she just went nuts with decorating and holiday foods, events, etc. When you go through the motions, your living space will feel like a happier place; and it'll pass the time.

And if you live somewhere with more of a mixed population, try to find info on your local cultural groups, and keep close tabs on their event calendars - and actually show up! India does a 'festival of lights' thing for example that has nothing to do with Santa or Jesus, but is still a super colorful, fun, and fucking DELICIOUS event. Most cultures have some kind of winter holiday, and when they host local events it makes for some really cool new experiences.

There's also some good ol' fashioned escapism. Pick your poison - reading, gaming, etc. Gaming is mine, and this is a good time of year to break out the nostalgic/comfort games. I just started another run of Subnautica, since it's basically a tropical diving simulator; get immersed into that, and you'll stop caring about the gloomy weather n' shit.