When a month has two full moons, the second one is called a blue moon.
This blue moon cycle is 2.72 years, or roughly every third year. This is not at all unusual, but when it happens on New Year's Eve, as it will on December 31, 2009 in the Western Hemisphere, this is a bit rarer as it is a 19-year occurrence. The last such one occurred in 1990, the next one will not be until 2028.
Australia and Asia, being in the Eastern Hemisphere, will not experience it until January 1. On New Year's Eve, with a new decade being introduced, there will also be a partial lunar eclipse. This not be visible in the Western Hemisphere but will be viewed by people in the Eastern Hemisphere.
Chances are the moon viewed on the night of December 31, 2009/January 1, 2010 will not be blue in the meterology sense, but it will qualify for the "Once in a blue moon" title.
www.huffingtonpost.com/wires/2009/12/29/rare
http://en/wikipedia.org/wiki/Blue_moon