It will be the second time we have seen him full during the month of December (the first time was on December 2). That phenomenon is commonly called a blue moon.
The occurence is rare (once in a blue moon!) because the Moon takes 29.5 days to complete its orbit around the Earth, making its journey match up almost perfectly with the length of a calendar month.
The last blue moon was in May of 2007. The last time a blue moon fell on New Year's Eve was in 1990. The next time a blue moon will end the year will be in 2028.
Does it mean anything out of the ordinary will happen? Will it cause your champagne to be even bubblier? Will everyone on Times Square suddenly grow tufts of hair on their chests? Will the Moon actually appear blue?
No, no, no and no.
"Once in a blue moon" originally meant exactly what the statement means, figuratively, to us today - a rare occurence. It made no reference to any sort of happening involving the Moon in the sky.
A writer for Sky and Telescope takes the blame for attaching an additional meaning - back in 1946 - and by mistake! Since then, two full moons in a single calendar month has been called a blue moon.
Will this extra full moon in December make this New Year's Eve even crazier than usual? Numerous studies have been conducted regarding a full moon's impact on pregnancies, emergency rooms, car accidents, etc. And while some statistics seem to suggest humans may act with less inhibitions on these nights (much like a werewolf?), no one has come up with any definitive proof.
Maybe things get crazy on the night of a full moon because it is so bright, especially on a cloudless night, encouraging people to head outdoors and, um, get into trouble?
Finally, a blue moon is not blue. The Moon is actually grey; it only turns different colors to us on Earth because of its light travelling through our molecule-rich atmosphere. That can make it appear yellowish, reddish, brown - but rarely blue.
By the way, because of different time zones on Earth, some parts of the world will not see this full moon until New Year's Day - meaning they will actually experience a blue moon during January of 2010. That will happen for people in Australia and Asia.
Read a transcript of Matt's Deep Six audio podcast about the "Origin of the Moon."