Sure. Though I don't know everything. Someone else might be better for that. *She stretches out.* So basically this one religion had this festival back in the ancient whatever-days, where they sacrificed a bunch of pigs and went around playing games and setting up role-reversal things, like for example if some rich guy had slaves, since that was what people did back then, they would serve their slaves food instead of the other way around, or women would dress up like men. And then everyone got drunk and went into food comas and sacrificed even more pigs.

Then this other religion, which was just this little splinter of an older one back then but is a lot bigger nowadays, decided it needed more followers and went "hey, we're stealing your holidays and making them about our god." And the Romans were all "okay, sure, do what you want, it's a stat holiday anyway." So they started saying December 25^th was celebrating the birth of God's son or whatever even though everyone knows he was actually born in March. If you want to know what's up with that, ask someone who took more than one religious studies course.

Well, in the end, people celebrate Christmas by rigging up the lights, putting up trees, giving each other presents, going shopping again the next day to get themselves everything they really wanted but never got, and drinking and going into food comas. No pig sacrifices, though. At least I've never seen anyone do it.