If we go back in time not too far, just a couple of hundred years, then we can be certain that much of the West was Christian. This wasn’t just a matter of ascription, as religion and the law were strongly intertwined. In other words, it wasn’t that people were Christian, so much as the whole culture was from top to bottom — but especially at the top.
Unsurprisingly, the word “holiday” originally came from the Old English hāligdæg meaning “holy day.” That is, originally the days were all religious observances.
Not only did people “get” the day off, they were required by law and religion to not work. And maybe more interesting, the idea wasn’t so much about partying as performing religious observances.
The whole idea of not working on holidays comes from its religious origin. Going back to Christianity, one of the rules is “keeping the sabbath day holy.” In the Middle Ages, this was extended outwards to other days of religious observance. Apparently it was used to good effect by the church in making wars more difficult.
Since, in many ways, the Church only had power over “holy” matters, they made an end-run around secular authority and would declare certain days holy so that they could make the warlords stop fighting. That is, they would take certain “feast days” into their sphere of control so that they could stop, or at least slow, wars.
That, combined with the idea of keeping every Sunday work-free, has come down to us as government-recognized holidays. So we can thank the Church’s interest in preventing useless bloodshed for the days off we now enjoy. Enjoy your days off, and Merry Christmas!