The winter solstice is one of the four quadrature points of the earths orbit, the others being vernal equinox, Summer Solstice, and the autumnal equinox. During the winter solstice, which occurs on or about December 22nd the night hours, begin to overtake the day hours and we begin to enter the time of darkness. According to ancient beliefs this is when the celestial ruler father sun took command and the days began to grow longer.
The winter solstice celebration was used by early Christian missionaries incorporating many of its customs and symbolisms into the faith to help gain acceptance. The birth of the celestial sun was also the birth of the sun of god. The harvesting of the Yule log was evolved into the tradition of a modern day Christmas trees.