Yule

In the Northern hemisphere, the winter solstice has been celebrated for millenia. The Norse peoples viewed it as a time for much feasting, merrymaking, and, if the Icelandic sagas are to be believed, a time of sacrifice as well. Traditional customs such as the Yule log, the decorated tree and wassailing can all be traced back to Norse origins.
The Celts of the British Isles celebrated midwinter as well. Although little is known about the specifics of what they did, many traditions persist. According to the writings of Pliny the Elder, this is the time of year in which Druid Priests sacrificed a white bull and gathered mistletoe in celebration.

Winter festivals were also common in Greece and Rome, as well as in the British Isles. When a new religion called Christianity popped up, the new hierarchy had trouble converting the Pagans, and as such, folks didn't want to give up their old holidays.

Christian churches were built on old Pagan worship sites, and Pagan Symbols were incorporated into the symbolism of Christianity. Within a few centuries, the Christians had everyone worshipping a new holiday celebrated on December 25.

In some traditions of Wicca and Paganism, the Yule celebration comes from the Celtic legend of the battle between the young Oak King and the Holly King. The Oak King, representing the light of the new year, tries each year to usurp the old Holly King, who is the symbol of darkness. Re-enactment of the battle is popular in some Wiccan rituals.