The final day of 2009 will be special for many reasons. It's a once in a Blue Moon occurrence--literally. A blue moon occurs on average once in every 2 0r 3 years. It is generally regarded as having two full moons in the same calendar month. This December 2009 is that month.
Even more interesting, the last time a blue moon occurred on New Year's Eve was 19 years ago in 1990. The next time it is expected to occur on New Year's Eve is 2028.