Despite our differences in social upbringing, nationality, language, religion, etc... We all agree in 1 thing, that Die Hard is a Christmas Movie, right?
Yippee Kay Yay... (The only movie that doesn't depresses me in Xmas)
Diehard is 100% a Christmas movie, however, Die Hard 2 which also takes place at Christmas time, is NOT a Christmas movie, as the fact that it's Christmas is coincidental and not at all integral to the plot.
Christmas being part of the plot is irrelevant - The Wizard of Oz is a Christmas movie. I think what makes Diehard a Christmas movie is simply that people like to watch it at Christmas.
About 75 years of tradition - until the 3 major TV networks lost their dominance it was always on during Christmas season. I don't watch them anymore so I don't know if they still play it.