From the Encyclopedia

"A winter festival was traditionally the most popular festival of the year in many cultures. Reasons included less agricultural work needing to be done during the winter, as well as people expecting longer days and shorter nights after the winter solstice in the Northern Hemisphere. In part, the Christmas celebration was created by the early Church in order to entice pagan Romans to convert to Christianity without losing their own winter celebrations. Most of the most important gods in the religions of Ishtar and Mithra had their birthdays on December 25. Various Christmas traditions are considered to have been syncretised from winter festivals including the following:
Saturnalia
In Roman times, the best-known winter festival was Saturnalia, which was popular throughout Italy. Saturnalia was a time of general relaxation, feasting, merry-making, and a cessation of formal rules. It included the making and giving of small presents (Saturnalia et Sigillaricia), including small dolls for children and candles for adults. During Saturnalia, business was postponed and even slaves feasted. There was drinking, gambling, and singing, and even public nudity. It was the "best of days," according to the poet Catullus. Saturnalia honored the god Saturn and began on December 17. The festival gradually lengthened until the late Republican period, when it was seven days (December 17-24). In imperial times, Saturnalia was shortened to five days.
Natalis Solis Invicti

The Romans held a festival on December 25 called Dies Natalis Solis Invicti, "the birthday of the unconquered sun." The use of the title Sol Invictus allowed several solar deities to be worshipped collectively, including Elah-Gabal, a Syrian sun god; Sol, the god of Emperor Aurelian (AD 270-274); and Mithras, a soldiers' god of Persian origin. Emperor Elagabalus (218-222) introduced the festival, and it reached the height of its popularity under Aurelian, who promoted it as an empire-wide holiday.

December 25 was also considered to be the date of the winter solstice, which the Romans called bruma. It was therefore the day the Sun proved itself to be "unconquered" despite the shortening of daylight hours. (When Julius Caesar introduced the Julian Calendar in 45 BC, December 25 was approximately the date of the solstice. In modern times, the solstice falls on December 21 or 22.) The Sol Invictus festival has a "strong claim on the responsibility" for the date of Christmas, according to the Catholic Encyclopedia. Several early Christian writers connected the rebirth of the sun to the birth of Jesus. "O, how wonderfully acted Providence that on that day on which that Sun was born . . . Christ should be born," Cyprian wrote.
Yule

Pagan Scandinavia celebrated a winter festival called Yule, held in the late December to early January period. Yule logs were lit to honor Thor, the god of thunder, with the belief that each spark from the fire represented a new pig or calf that would be born during the coming year. Feasting would continue until the log burned out, which could take as many as twelve days. In pagan Germania (not to be confused with Germany), the equivalent holiday was the mid-winter night which was followed by 12 "wild nights", filled with eating, drinking and partying. As Northern Europe was the last part to Christianize, its pagan celebrations had a major influence on Christmas. Scandinavians still call Christmas Jul. In English, the Germanic word Yule is synonymous with Christmas, a usage first recorded in 900"