QUOTE(emberfly @ Feb 3 2009, 05:45 PM)

I haz quetshunn.
Why do we celebrate Jesus's birthday on December 25? I think it's safe to say that everyone knows that he wasn't born on December 25.....
Marketing, pretty much.
Around the time Christianity was getting its start, the cult of Mithras was incredibly popular in the Roman Empire. Mithraism is, by all accounts, a pretty f*cking awesome religion; many of its celebrations involved a lot of drinking and orgies, things that would really let us all cut loose today. Anyway, in order to compete with Mithraism, Christianity's founders realized it had to offer something different, so it went the opposite direction and claimed that a lifetime of piousness and modesty would allow followers to reap great rewards in the afterlife. (Christianity was, in fact, one the earliest cults -- and certainly one of the most high-profile -- to preach that restraint now would lead to eternal salvation after death. Most other religions certainly preached about the afterlife, but very few cast it in terms of "heavenly perfection vs. a pit of boiling lava".)
Anyway, around December, followers of Mithraism held a huge festival to celebrate the end of the harvest season, which, of course, involved a lot of drinking and sex. Christianity wanted to be different, but its followers knew that they had to have an equally awesome celebration in order to draw supporters. So, in keeping with the theme of eternal salvation, they cooked up this festival that honored their "god", of sorts; however, instead of wine and orgies, they concocted this scheme in which they convinced people that worshipping this baby
now would, of course, lead to something much better down the road. Essentially, you had you choice: drunken revelry now but eternity in hell, or modesty now and eternity in heaven. As it turned out, they (slowly) one a fair number of converts with this pitch.
I'm sure there're other reasons that led to Christmas as well, but the rivalry with Mithraism was most likely a pretty big factor.