Is it? (It’s been decades since I watched it!) Aren’t they in the Nakatomi Plaza (?) for a Xmas party and that’s the only mention of Xmas?
John and Holly are both there because it’s Christmas. The central narrative is that he’s using it as an opportunity to try to reconcile his failed marriage, it’s his motivation throughout the film. Plus there’s multiple references: Christmas music, people wanting to get home to their families, isn’t there even a Santa? And of course, “Holly” herself.
Sure, there could have been a different framing, but equally 101 Dalmatians could have been about cats.
By contrast, in Lethal Weapon it’s barely mentioned as far as I remember. Though likewise, it’s decades since I watched that. An obvious solution presents itself…