|Roman Imperial repoussé silverdisc of Sol Invictus (3rd century), found at Pessinus (British Museum)|
Saturnalia was originally a holiday created by Emperor Augustus to celebrate Saturn, on 17th December -- my birthday! It then developed into a week-long festival, the craziest week in ancient Rome, where people made merry and even slaves were allowed to indulge in excesses that would have been punished by death at any other time.
The date coincides, closely enough, with the shortest day of the year (which the Romans believed to be 25 December whereas we know it is 21 December). Light prevails over darkness and days start getting longer again, an occasion to celebrate indeed.
Then the Christians took it over during the reign of Emperor Constantine, who had accepted Christianity as a religion of the Empire. The Church decided that Christ had chosen to be born on the shortest day of the year, after which light again starts to prevail, to symbolize his contribution to the rebirth of humankind.
I feel it's too bad that the ancient tradition of Saturnalia is gone. Not so much for the sake of Saturn, of course. But rather for what it symbolized: fun and naughtiness for a week but strict rule of Roman law for the whole year!