The traditional time for celebration, 25 December, coincides with pagan festivals and follows closely on from the winter solstice, when the Sun reaches its greatest excursion south of the Equator. Late December marked an important turning point in the Sun’s apparant course, as the daily quota of sunlight grew longer and stronger. Since ancient times, people have lit candles, bonfires and yule logs to help nourish the sun god when he was at his weakest and drive winter and its hardships away. The Roman festival of Saturnalia in mid-December is one example. In keeping with its name – saturnus means ‘plenty’ or ‘bounty’ – the celebration involved feasting, gambling, dancing and singing in honour of Saturn, the Roman god of agriculture. Hats were worn, though not paper ones. At these festivals, the master served the slave, a ritual that can still be seen at office parties today. Gifts were also given, including branches of sacred wood (these evolved into the switches left to punish bad boys and girls in some seasonal festivities). This festival was swiftly followed by the Kalends, a new-year celebration. Already we can see the genesis of the celebrations we enjoy today.
Fra Can Reindeer Fly? : The Science of Christmas av Roger Highfield.