Few posts up, Gaazy.
December 25th was also the Feast of Mithras, which was yet another old religion the early Christians wanted to stomp out by co-opting the existing holiday.
Basically, all the peasants were technically converted to Christianity, but kept celebrating their old festivals, since it was traditional. (and since none of the other gods they'd worshiped cared about honoring other gods too). Christianity, on the other hand, has a serious stick up its ass about even the existence of other dieties, and so this pissed them the hell off.
When flat out telling people to stop those celebrations predictably didn't work, the Church got clever. It invented a Christian "holiday" that was dedicated to similar purposes as the existing one, and got people to at least alter the forms of their celebrations to the new one. Generation passes, and everyone has grown up celebrating Christmas instead of the Feast of Mithras, the new holiday has taken over, and the old one vanishes.
That's not the only holiday they tried that with, obviously. Take a look at a lot of Christian holidays, and marvel at how they seem to mysteriously line up with the equinoxes and solstices, which people had celebrated for millenia, because they marked important times of the year. (start of spring, beginning of winter, etc. etc. etc.) The fact that they line up so closely in so many cases is NOT an accident, I assure you.
-Arlos