Serious question. Do you really consider Die Hard a Christmas movie, or are you just trolling?  It seems like this topic cropped up about 5-10 years ago, for no reason whatsoever, and it just seems so silly.  
I will not die on this hill, but Die Hard is absolutely not a Christmas movie for the following reasons: 
- It was released in July, obviously not Christmas season. 
- Christmas was not a central part of the plot, only a small element of the plot. 
- it’s not traditionally shown around Christmas, at least on the stations that I watch.  
- It was never promoted or themed as a Christmas movie. 
- It’s not really family friendly.  You wouldn’t sit down and watch it with your 5 year old kids.  “Yippie Kiyeah Motherfucker” 
- None of the sequels were even remotely associated with Christmas. 
We could go down the list of lots of movies that could be a Christmas movie, but somehow this one always makes a lot of conversation. What about Gremlins?