saturnalia is actually roman feast dedicated to saturn, only originally lasting one day (dec 17) but then later extending a whole week, until the 23rd.
excerpted from wikipedia,
The Saturnalia was a large and important public festival in Rome. It involved the conventional sacrifices, a couch (lectisternium) set out in front of the temple of Saturn and the untying of the ropes that bound the statue of Saturn during the rest of the year. Besides the public rites there were a series of holidays and customs celebrated privately. The celebrations included a school holiday, the making and giving of small presents (saturnalia et sigillaricia) and a special market (sigillaria). Gambling was allowed for all, even slaves; however, although it was officially condoned only during this period, one should not assume that it was rare or much remarked upon during the rest of the year. It was a time to eat, drink, and be merry. The toga was not worn, but the pilleus (freedman's hat) was worn by everyone. Slaves were exempt from punishment, and treated their masters with disrespect. The slaves celebrated a banquet: before, with, or served by the masters. A Saturnalicius princeps was elected master of ceremonies for the proceedings. Saturnalia became one of the most popular Roman festivals which led to more tomfoolery, marked chiefly by having masters and slaves ostensibly switch places. The banquet, for example, would often be prepared by the slaves, and they would prepare their masters' dinner as well. It was license within careful boundaries; it reversed the social order without subverting it.
The customary greeting for the occasion is a "io, Saturnalia!" — io (pronounced "yo") being a Latin interjection related to "ho" (as in "Ho, praise to Saturn").
practically edible says,
People donned red, soft woolen caps (which seem to be reminiscent of Santa's Hats, and the hats we pull out of Christmas crackers), and decorated their houses with boughs of greenery. Courts and schools were closed, as well as many businesses. People spent their time gambling and feasting. Public banquets seem to have started by 217 BC. Lords of Misrule (Saturnalicius princeps) were appointed for the festivities and feasts. In wealthy houses, it was a time of role reversal, when masters would wait on their slaves.