A lunar month takes 29.53 days. This is the amount of time it takes for the Moon to complete a cycle through all the phases, from new moon to full moon and then back to new moon again. This is very close to the length of a month in the Western calendar, which usually have 30 or 31 days. Every month usually has one of each of the phases. So a typical month will have a new moon, first quarter, full moon and last quarter moon. But every now and then, a month will have two of the same phases. When a month has two full moons, the second one is called a "blue moon".
Blue moons are rare, and that's where the phrase comes from, "once in a blue moon". How rare? They always happen once every 2-3 years (2.72 years, to be exact).
So when is the next Blue Moon? December 31, 2009!
The name of the first Full Moon after Yule is sometimes known as the "Wolf Moon" or "Old Moon."
This should make this night very special, not only a full moon on New Year's Eve but the Eve of a new decade!
What would you do that is "once in a blue moon"?