Magus,
That would make you older than I. I believe Christmas was decided to be celebrated on December 25th in 336 CE. Before that it was not an official celebration at all.
The Epiphany and Easter were the most important holy days of the church. The date of December 25th was likely chosen to coincide with the Roman celebration of Saturn (the god of agriculture) and the Persian festival of Mithra (the god of light). This made it easier for pagans to accept Christianity.
Edit: December 21st is the winter solstice - although it varies plus or minus a day or so. I believe most Pagans still celebrate the winter solstice as the mid-winter festival. For some it is one of two important days of the year, as days get longer and nights get shorter.