While I don't exactly use it in any game worlds I DM at the moment, (Sadly, Zero), I do have a free-form character who celebrates the winter solstice.
He celebrates it by locking himself in the safest, most fortified place he can possibly find. On the day of the longest night, the creatures of darkness are the most powerful, and it is on this night that they claim the most souls. It is on the morning after, the Day of the Survived, that surviving children are finally given names, that the new year is celebrated, and it's frequent on that same day after that marriages are made/proposed.