I swear for years I thought it was something with boxing the sport. I have some memory of seeing a movie, maybe something like Snatch or Lock, Stock and Two Smoking Barrels, or something where they talk about boxing day as the day that 'things go down' and then the next day they're at a ritzy boxing event.

But probably I just jammed that memory together from who knows where.

edit: Yes, punish me for not knowing something, the entire point of this sub. I don't quite feel properly chastised yet.