Along with all the griping going on about the so-called "War on Christmas," we've been seeing a lot of wagging fingers reminding us at every opportunity to remember the "Reason for the Season," and inevitably, these folks are referring to Jesus. The truth is, not very many of the traditions associated with Christmas are Christian- or have anything to do with the birth of Jesus.
So how did a raucous Pagan celebration become associated with Christianity? Very intentionally.* At the time the date of Jesus birth was officially assigned as Christ's birthday, Rome was pretty thoroughly Christianized, but the former pagans were more reluctant to give up the holiday than they were their gods. After brushing aside inconvenient facts (such as the bible's own accounts placing the birth of Jesus in the spring), the names were changed- but the holiday remained the same. Elements of popular Pagan nativities were borrowed to create a backstory fit for the new god, from the heralding star to the shepherds visiting the newborn in his cave.
Updated: I've changed the link to a much more well written article.