The Before Yule Moon is the full moon which occurs before the feast of Yule on December 21st. This year that Full Moon falls on December 19th.
There are twelve full moons in a year each has its own name. These names go back over 1,000 years to the times of the monasteries. The monks watched the natural cycle of life including the night sky and divided the twelve full moons into the four seasons.
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. This is known as Sol Invictus (The Undefeated Sun). It was originally a pagan festival. It is possible that after Constantine the Great became Emperor of the Roman empire in 324 CE and converted to Christianity, that he then merged this pagan feast with a Christian celebration.
Christmas Day was not celebrated on December 25th until 336 CE.
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. They would use some wood from the previous year as kindling to light their fire.
Today we often see the American names for the full moons being used. I however prefer to use the old English names.
Today of course the Yule log has turned from firewood into a cake!!