The full moon on December 15th is known as the Before Yule Moon. The feast of Yule occurs on the night of December 21st which is the day of the year when the Sun is at its lowest in the sky and produces the shortest period of daylight of the year. This date can vary from year to year by a day or so.   A long time ago people watched for the full moon in December; they then had from that date until the feast of Yule to chop down a Yule log from the forest in readiness to burn it from the feast of Yule for twelve nights. Today of course the Yule log has turned from firewood into a cake!!