Die Hard is a Christmas movie. At least that seems to be the line, or the joke. But skeptics say, "It's not really festive." So, what is a Christmas movie? Most lists of Christmas movies start with It's a Wonderful Life. But 90% of that film doesn't happen around any specific holiday, and really isn't very festive. Meet Me In St. Louis is shown frequently on TCM this time of year - but it's probably as much a Halloween movie as it is a Christmas movie - if we're just going by screen time. Based on holiday settings, I guess we should all be happy that Stanley Kubrick made a Christmas movie....
But this is probably the fun of defining "Christmas" movies. There are Christmas westerns (3 Godfathers), noir (Blast of Silence), war (Joyeux Noel), musicals (White Christmas), etc... Pretty much every genre has a Christmas take - there's even Christmas movies for the non-holiday season (Christmas in July).
I'm not really sure what my favorite Christmas movie is. But the next time someone asks me, my response should probably be "in what genre?"