Saturnalia was an ancient Roman festival in honour of the deity Saturn, held on 17 December of the Julian calendar and later expanded with festivities through to 23 December.
The holiday was celebrated with a sacrifice at the Temple of Saturn, in the Roman Forum, and a public banquet, followed by private gift-giving, continual partying, and a carnival atmosphere that overturned Roman social norms: gambling was permitted, and masters provided table service for their slaves. The poet Catullus called it “the best of days.”
In Roman mythology, Saturn was an agricultural deity who was said to have reigned over the world in the Golden Age, when humans enjoyed the spontaneous bounty of the earth without labour in a state of innocence. The revelries of Saturnalia were supposed to reflect the conditions of the lost mythical age, not all of them desirable. The Greek equivalent was the Kronia.
Although probably the best-known Roman holiday, Saturnalia as a whole is not described from beginning to end in any single ancient source. Modern understanding of the festival is pieced together from several accounts dealing with various aspects. The Saturnalia was the dramatic setting of the multivolume work of that name by Macrobius, a Latin writer from late antiquity who is the major source for information about the holiday. In one of the interpretations in Macrobius’s work, Saturnalia is a festival of light leading to the winter solstice, with the abundant presence of candles symbolizing the quest for knowledge and truth. The renewal of light and the coming of the new year was celebrated in the later Roman Empire at the Dies Natalis of Sol Invictus, the “Birthday of the Unconquerable Sun,” on December 25.
The popularity of Saturnalia continued into the third and fourth centuries AD, and as the Roman Empire came under Christian rule, some of its customs have influenced the seasonal celebrations surrounding Christmas and the New Year.
(Just like Sheldon said)