Io, Saturnalia: In ancient Rome, autumn planting was completed before the winter solstice (December 25 in the Roman calendar, December 21 in ours) and in the week before the solstice it was time to ask Saturn, the god of seed and sowing, to start lengthening the days again so the seeds would grow into a fine spring crop. After the temple sacrifice or the home pre-cenaeal prayers on the evening of December 17, the exhortation "Io, Saturnalia" was the signal to start serious partying in honor of the god. Saturnalia started as a one day celebration but soon became a seven-day party and the most popular of the ancient Roman festivals. A "Lord of Misrule" was selected in each household (still done in some British Christmas celebrations) and jokes and pranks were the order of the day. Heavy eating and drinking and especially procreation were encouraged. Small gifts were exchanged on the last day of the festival -- kids got dolls and adults exchanged candles. It was no coincidence that the early Christian church in Rome, trying to diminish December licentiousness, chose the 25th of that month to celebrate Christmas. For more on Saturnalia, go to:
http://www.ku.edu/history/index/europe/ancient_rome/E/Roman/Texts/Macrobius/Saturnalia/home.html Macrobius on Saturnalia.
http://www.cs.utk.edu/~mclennan/BA/Saturnalia.html to see what modern pagans think you should do for the holiday.
http://ancienthistory.about.com/cs/saturnalia/ Saturnalia links from About.com.