Just because Die Hard takes place at Christmas does not make it a Christmas movie. A Christmas movie has something central to the plot ABOUT Christmas. If you can replace a single plot-device with literally any other gathering (say...a company holiday party that inexplicably takes place on Christmas Eve with like...a birthday...or a July 4th cookout), it is not enough to make it a Christmas movie.
It is a mediocre action movie that KINDA uses Christmas as a plot device (but doesn't even use it well, IMO).