The original Saturnalia was a Roman mid-winter festival held in the middle of December, starting on the 17th in the modern calendar (or the 25th in the Roman one). It lasted for seven days and was a period when excess was encouraged: the shops were closed, gambling was permitted, presents were exchanged, slaves were given licence to speak their minds and join in the fun, and generally joy was unconfined. The holiday began with a sacrifice to the Roman god of agriculture, Saturn (Latin satus means sown), whose day it was.
From the eighteenth century on, the word became a more general one in English for a period of unrestrained licence at any time of year, often with a lower-case initial letter. An example of modern use is in Joseph Heller’s Catch-22 of 1961: “Other men picked up steam as the hours passed, and the aimless, riotous celebration continued. It was a raw, violent, guzzling saturnalia that spilled obstreperously through the woods to the officers’ club and spread up into the hills toward the hospital and the antiaircraft-gun emplacements.”
The name of Saturn has also given us Saturday (Saturni dies in Latin, the day of Saturn) and saturnine, gloomy, dark featured, dull, and moody — a description that sits oddly with the revelry of his annual festival. But the medieval alchemists identified the planet Saturn — named after the Roman god — with the element lead and astrologers linked it to slowness and gloom.