When it's not a "Christmas movie," but there's Christmas music on the soundtrack.
I'll start with this one, which I'm now playing:
Mommie Dearest ~ Henry Mancini (La-La Land Records)
18. Christmas Music (2:06) Contains "Away in a Manger" composed by James E. Epilman and Martin Luther, and "Silent Night" composed by Franz Gruber; arranged by Jack Hayes.
Not a film, but a VERY odd place for Jingle Bells to pop up -- Shostakovich's incidental music to King Lear. He uses the melody as one of the 'Songs of the Fool'.
My least favorite: "Do You Know How Christmas Trees Are Grown?" John Barry OHMSS What was he thinking? Does anyone else here think it was a joke?
A joke? No. A perfect piece of music for the scenes in the film? Yes ...
... and a highly enjoyable Christmas song of which I never tire.
I acknowledge that as a teenager when I first purchased the album I wasn't too taken but the song has matured with me over the years and I think it's a lovely track.
Those of you who are into obscure Christmas-themed films owe it to yourselves to watch the very fine 1945 2-reel Warner Bros. Oscar-winning short, STAR IN THE NIGHT, starring J. Carrol Naish and directed by Don Siegel, with music by William Lava.
This short runs periodically on TCM between features, and particularly during the Christmas season.
Battle of the Bulge (1965) – Frankel’s ”Christmas in Ambleve-The Americans Debate” track briefly but beautifully incorporates “Good King Winceslas” and “The First Noel” in the opening moments before segueing into music associated with Maj Kiley, the Panzers and more somber music with overtones of the impending destruction of the city. The juxtaposition of the cheerful Christmas music with the somber really sets the mood for things to come abd is a high point in the music.
Most of Shane Black's movies are set at Christmastime and would be eligible, but the best example I can think for this is the Jingle Bell Rocking opening of LETHAL WEAPON.
Frank Sinatra sings "Have Yourself A Merry Little Christmas" on Side 1-Track 7 (this track is not on the FSM CD):
"Silent Night" on Side 2-Track 1:
"Have Yourself a Merry Little Christmas" is heard in "The Victors" as an American WWII deserter is being led to his death by firing squad. The song is also used in a similar way in "The Godfather", as Corleone hit-man Luca Brasi prepares to pay a visit to the Tattaglia family, where he will be strangled to death.