Saturnalia was the Roman mid-winter festival introduced around 217 BCE to raise citizen morale after a crushing military defeat. Originally celebrated for a day, on December 17, popularity grew it to week-long extravaganza where drinking and debauchery started on the shortest day and continued through the longest night. The customary greeting for the occasion is a "Io, Saturnalia!" (pronounced "yo") The festival harks back to the Golden Age of Man when Saturn ruled and all men were equal, there was no work, and everyone enjoyed peace and happiness.

Saturnalia, the festival, was a time of celebration. It was marked by tomfoolery and reversal of social roles, in which slaves and masters switched places . The toga was not worn, but rather colorful, informal "dinner clothes"; and the pileus (freedman's hat) was worn by everyone. It was a time of giving gifts such as candles and traditionally small clay earthenware figures called sigillaria. which were flat, oval faces, of self-setting clay, with a hole for hanging in each one, so that later, for the Roman God Dionysia, they can be hung on a pine tree. The pottery represented human heads once placed on the god's altar. Some of the gifts given were dice, knuckle bones, moneyboxes, perfumes, pipes, a pig, sausage, a parrot and with passing time more elaborate gifts such as silver. There was also a darker side to the Saturnalia festival. In the Danube , one of the long-standing frontiers of the Roman Empire , Roman soldiers would choose a man from among them to be the Lord of Misrule during Saturnalia and at the end slit his throat on the altar of Saturn

In the fourth century, the Roman Emperor Constantine designated December 25, Sol Invictus (birthday of the Sun-God Mithra), as the birthday of Jesus Christ, thereby placing the true Savior among the pantheon of Roman gods.  Constantine succeeded in drawing Christians into the pagan celebrations of Rome , which procured the religious unity needed for the success of the Holy Roman Empire . Saturnalia, Sol Invictus, and Christmas were merged into one.

Saturnalia poem

When Saturn rules, all things are turned around,
and everything becomes its opposite.

Just once each year this Image is filled up;
it's empty while Saturnus lies asleep.

We feed Him with the oil that's pressed from corn,
the golden nectar from the nuggets born.
So also we in wisdom store away
our energy to use another day.

(mostly cut and paste editing from Wikipedia and other Saturnalia web-sites)