According to Macrobius (I think it was) what was originally a one day festival (17 December) got expanded to seven days in part due to the calendar change introduced by Julius Caesar in 9955 HE (46 BCE) on the advice of the shadowy Sosigenes of Alexandria. You see, he expanded December from 29 to 31 days and thus threw off the date of Saturnalia, which was originally fixed at 14 days before the Kalends of January, but then changed to 16 days before the Kalends. Some people continued to celebrate the XIIII Kal Jan date, now 19 December, while others the XVI Kal Jan date (17 December), and with two dates for Saturnalia it’s easy to see how the 18th got thrown in as a kind of bonus, like the Friday after Thanksgiving in the good old USA. But this doesn’t explain the extension for another four days, unless maybe people just plain felt that after getting the autumn field work done, it was time for a party.
It matters not. Personally I don’t trust ancient explanations of ancient feasts; they all have the stench of ad-hocery about them. I doubt very much that the ancients knew that much more about them than we do; their origins were probably as lost to them as to us.
The thing about Saturnalia, though, is the evocation of a long-lost Golden Age, presided over by Saturn, where distinctions of rank did not exist, where the earth gave forth its abundance without the need of labor, where justice reigned. A time before Prometheus brought fire to man or Pandora opened that goddamn box.
In a way, I suppose, there was a golden age. Gold is one of the easiest metals to work, and one of the first discoveries in metallurgy must have been the magic of gold. It’s not the most useful of metals, but damn is it pretty. And it’s not like the other rocks. The discovery could even have been pre-agricultural, when hunters and gatherers roamed the earth, and division of labor was pretty much restricted to the gender division that humankind seems to have had from before the beginning. Distinctions of rank may have depended on who was the strongest, or who had the most success in the hunt, or the gather, or whatever. A golden age of sorts, though not exactly, well, Eden.
The mythical golden age is much cooler, and it’s hard to fault the attempts to recreate it with that peace-on-earth good-will-toward-men spirit that was the stuff of Saturnalia. Present-giving, candle-lighting, gambling, free speech, masters waiting on their slaves—good times, good times. But it isn’t real, and when Saturnalia ends, all that stuff goes back in the box till the next year. Still, as Statius observed:
For how many years shall this festival abide! Never shall age destroy so holy a day! While the hills of Latium remain and father Tiber, while thy Rome stands and the Capitol thou hast restored to the world, it shall continue.