WSJ has a brief history of the holiday, from Roman times, through the middle ages when it was more like a modern new years, to the reformation where the holiday was largely forgotten to the 19th and 20th centuries that created the child-oriented holiday we know celebrate.
The Christmas of parties and presents is far older than the Nativity. Most ancient cultures celebrated the winter solstice, when the sun reaches its lowest point and begins to climb once more in the sky. In ancient Rome, this festival was called the Saturnalia and ran from Dec. 17 to Dec. 24. During that week, no work was done, and the time was spent in parties, games, gift giving and decorating the houses with evergreens. (Sound familiar?) It was, needless to say, a very popular holiday.
In its earliest days, Christianity did not celebrate the Nativity at all. Only two of the four Gospels even mention it. Instead, the Church calendar was centered on Easter, still by far the most important day in the Christian year. The Last Supper was a Seder, celebrating Passover, which falls on the day of the full moon in the first month of spring in the Hebrew calendar. So in A.D. 325, the Council of Nicea decided that Easter should fall on the Sunday following the first full moon of spring. That’s why Easter and its associated days, such as Ash Wednesday and Good Friday, are “moveable feasts,” moving about the calendar at the whim of the moon.
A Brief History of Christmas – WSJ.com