In the third century of the Common Era (traditionally known as AD - now CE), the early Christian church co-opted the winter solstice. What WAS the winter solstice?
The year 336 AD (now known as CE) is the earliest record of the early church celebrating Jesus' birth on December 25. (This of course doesn't mean that 336 is the first time it happened. It only means that we don't have records of it taking place before 336). Nevertheless, prior to the church picking December to celebrate Christmas, the ancient world celebrated the Winter Solstice.
Follow the link to read what National Geographic writes about this ancient festival:
http://news.nationalgeographic.com/news/2002/12/1220_021220_solstice.html
