Feast of Fools

"It is now the month of December,
when the greatest part of the city is in a bustle.
Loose reins are given to public dissipation..."

Seneca, AD 50

December 17th was date for a popular Roman festival called Saturnalia. For one day, slaves had equality with their masters and there was lots of eating, drinking and shenanigans. Scholars say that some aspects of Saturnalia are present in modern day Christmas customs, namely gift giving.

The practice of giving gifts in December was suppressed (not very successfully) by the Catholic Church in the Middle Ages, due to "pagan" origins. By medieval times, the festival of Saturnalia had evolved into the Feast of Fools, a day of rather blasphemous extravagances and free speech. Naturally, it was roundly condemned by the medieval church.
In England, the Lord of Misrule was often a peasant who was appointed to preside over the wild partying, in the pagan tradition of Saturnalia. All the ordinary rules of life were reversed and masters served their slaves. The Lord of Misrule had the power to command anyone to do anything for this short period. It was like a brief social revolution, and everyone blew off some steam.

Santa Claus might have descended from the mock king who held court at Saturnalia, or the Lord of Misrule who presided over the yuletide Feast of Fools in the Middle Ages. The ceremonies often mocked the highest offices of the church, and this lewd reveling gave vent to underclass hostility toward feudalism and the all-powerful church. The Feast of Fools was finally forbidden under severest penalties by the Council of Basel in 1431. It didn't work-- in Europe the festival was enjoyed until the 18th century.