| Considered by the Christian Church to be an indecent Pagan festivity, they did whatever they could to get rid of it. Unfortunately, it was incredibly popular for obvious reasons (and Webster 1913 does a good job of making it seem quite tame), and so they assimilated it into what's now known as Christmas, except they replaced the sex with the exchanging of gifts. Joy.
In any case, it's generally believed that Saturnalia has its roots in nature worship (rather than in Roman tradition); specifically, it was a celebration of the Winter Solstice, during which Father Time (who has been bastardized into Santa Claus) would come to deliver the goods, as it were, to Mother Nature, fertilizing the earth. It's described rather well at Electric Sheep; at the time of this writing it is specifically at http://www.e-sheep.com/Saturnalia/. The author of this somewhat silly recounting recommends "The Winter Solstice: The Sacred Traditions of Christmas" by John and Caitlin Matthews for the purpose of further research, as well. | 
