Saturn's great festival, the Saturnalia, opened on the seventeenth of December and ran for several days, and it was the most beloved holiday of the Roman year. For its duration the ordinary order was turned upside down: masters served their slaves at table, gambling was allowed in public, a mock king of misrule was chosen, gifts were exchanged, and the formal toga was set aside, a licensed reversal that recalled the equality of his Golden Age. Many of its customs of feasting and gift-giving passed into the later midwinter festivals of Europe. His temple in the Roman Forum, one of the oldest in the city, held the aerarium, the state treasury, so that the god of the abundant past also guarded the wealth of the present. Later thought made Saturn a figure of time and its devouring passage, an association that ran through astrology into the melancholy planet-god of the Renaissance.
Because the temple also held the state treasury, the aerarium Saturni, the phrase sub Saturno, under Saturn, could stand as shorthand for the public finances themselves, tying the god of a mythical age without private property to the very institution that managed Rome's money. The mock king of misrule chosen during the Saturnalia, sometimes called the Saturnalicius princeps, who issued playful commands his household had to obey, is widely cited by folklorists as an ancestor of the medieval and early modern Lord of Misrule and King of the Bean traditions of European midwinter festivity, one of the clearer unbroken threads running from Roman religion into later folk custom.