I believe some consider Die Hard to be a Christmas movie because the story occurs at Christmas Eve. A rather loose connection in my opinion, but that's just my view. Die Hard could have taken place at any time of year and it wouldn't have made any difference to the plot / action at all.
Personally, I consider a Christmas movie to be one where the time of year itself is central to the story or a significantly meaningful connection to Christmas exists.
"Love Actually" takes place in the weeks running up to Christmas and has a strong theme of people coming together for the holiday. The final sequence, in the arrivals lounge at Heathrow is really touching and leaves you with the sort of "feelgood" that Christmas films - again, in my opinion - should impart.
"It's A Wonderful Life" is a film where most of the story doesn't take place at Christmas, and yet, the story, throughout the film, is clearly intended to come to a climax on Christmas Eve. That's enough for me to consider it (as millions do) a "Christmas" film.
One film that was shown on British television at Christmas - and just about every other holiday throughout the year - for many, many years was "The Great Escape".... a movie that was about a WWII prisoner of war breakout. It had absolutely nothing whatsoever to do with Christmas or any holiday, but its appearance in the schedules - often at Prime Time - eventually became a national joke.
I guess a Christmas movie can be anything anybody wants it to be, but I know what I consider to be a movie for the holiday and I stick with that.
Take the movie Gremlins as an example. Gremlins could be considered a Christmas movie and yet oh no why it's so scary and violent with all the monsters. No frosty or baby Jesus or not one stitch of a shining star above the manger or hay or wise men or Mr Scrooge.
Alas, it is true. Christmas is whatever you say it will be.