A few people when i ask this think they are clever and say 'gremlins' 'die hard' etc, but there are also a lot of people that do actually call these movies xmas movies, which is fine, it's all subjective.
To me, a christmas film is a movie that only really works when played at christmas or there abouts.
I grew up with he Dudley Moore 'Santas claus the movie' then it went to Home Alone but i kinda watched that anytime due to the hi jinx!
Then Nightmare before christmas, Polar Express. It's not until recently i saw A christmas Vacation. Also, for me, a hristmas film i like it when it's nostalgic and reminds me of mine or has a great sound track - elf is one of those.