Saturnalia, as any student of Latin knows, was the Roman holiday analogous to Christmas. On XVI Kalends Ian (the sixteenth day before the first of January), the feast of Saturnalia was celebrated. Eventually, the celebration stretched to multiple days. As found on the University of Vermont Department of Classics website:
Similar to our Christmas, [Saturnalia] was characterized by the giving of gifts. In fact, eventually the rites of the Saturnalia festival was absorbed into the Christian tradition and reborn as Christmas. The social order was also inverted, citizens would serve dinner to their slaves, and those slaves would later go out into the streets and gamble with dice, which was illegal during the rest of the year. On the day of the festival itself, there was a sacrifice at the temple followed by a public banquet. After this banquet, citizens are reputed to have shouted “Io, Saturnalia!”
Now, Romans were known to love their holidays, but Saturnalia was their favorite. In fact, it was so embedded into the Roman culture that by the time Christianity had taken hold in the fourth century A.D., many (if not most) of the traditions of the Saturnalia had been absorbed into Christmas.