Dagger of the Mind - mentions an unseen Christmas party, the on-the-nose named "Helen Noel"
Devil's Due - a scene from A Christmas Carol
Tapestry - Picard gets the It's a Wonderful Life treatment.
Too bad there isn't one good Trek TV episode where Christmas is more than an event that happened offscreen, a literary allusion, a flashback, or a holodeck program, and some of the main crew characters celebrate it in some fashion.