Saturn was the oldest of the Roman Gods and father of many of them. For a long period of pre-history he ruled heaven and earth. His only fear was a prophecy that one day he would be overthrown by his children. He therefore developed the reasonable but unfortunate habit of eating his children shortly after their birth, to prevent the prophecy coming true. True he could have just worn a condom and avoided the whole cannibalism and infanticide issue but we can all sympathise that it's just not the same.
Eventually his wife Ops, who was worried that all the nappies she bulk bought years ago were going past their sell by date, managed to trick him and his son Jupiter (also known as Zeus) survived birth, grew to maturity and overthrew him.
Jupiter ruled Olympus and exiled Saturn, sending him far away from civilized Greece to backwards, rural Italia. Once he got there though, Saturn, the god of planting and harvesting, was determined to improve his reputation. He changed into a kindly, benevolent figure who cared for all the people of Italy. He ensured that crops grew so well that nobody needed to work in order to have all the food they needed. He led an almost socialist utopia where all men were equal and there was no war or division. It was a time remembered fondly by future generations who referred to it as "The Golden Age."
By the time of Julius Caesar and the ancient Rome we think of today, people thought they were living in "The Iron Age" - a time of rust and decay. Saturn had the oldest temple in Rome but was barely prayed to by comparison to mighty Jupiter or terrible Mars. Except that is, for one week in December - from the 17th to the 24th, a week of festivities.
This was the most anticipated week of the year, especially by the poor and the young. The festival was held in mid winter to encourage the God of harvests to bring back the times of plenty, and in his honour Rome, the home of proud emperors and noble senators, would embrace the egalitarian spirit of earlier times. All work would cease and the entire city would feast and exchange gifts for seven days, but the most shocking element was the social upheaval - masters would wait on their slaves at table and follow their orders. In each household one of the slaves of lowest standing would be appointed Lord of Misrule. His role was to make sure everyone was having suitably anarchic fun.
Admittedly all the slaves were very well aware that at the end of the week everything would be back to normal and their masters could have them killed or sent to the mines with absolutely no legal consequences so I'm sure people probably didn't get too wild but it sounds like fun doesn't it?
The early Christians thought so, which is why the festival was appropriated to celebrate the birth of Jesus, which as later generations of puritans pointed out, does not have a date of birth mentioned in the Bible. The practice of appointing a Lord of Misrule survived in England until Tudor times. However the points of both festivals are renewal and the celebration of light coming from darkness which is essentially the same thing.
One thing we should be grateful the Christians didn't take on board was the practice of legionaries garrisoned in Bulgaria - they chose to hark back to the earlier, darker worship of Saturn where blood sacrifice was needed to ensure Spring returned. Each November the soldiers chose a man to be their Lord of Misrule. He was given all the food, drink and women he could possibly desire for four weeks, and then on the feast of Saturnalia he was handed a knife and given a choice between cutting his own throat or being tortured to death. Which puts Brussels Sprouts in perspective.