"Blue moon 2009: a New Year's Eve rarity"

At the end of 2009, a  blue moon  will preside over the annual New Year’s Eve countdown and accompanying ball drop in Times Square.
But what does this mean?

For most people, it will just mean a full pie in the sky Thursday night. A blue moon is simply the second full moon in month. It doesn’t actually have anything to do with color at all.

Usually, there’s only one full moon each month because the monthly calendar was built on the lunar cycle 29.5 days). But once every 2-1/2 years, those extra half days add up to two full moons in a month.

So where does the name come from?

According to NASA's website, the term blue moon was "used in much the same way we use the term 'harvest moon.' There were twelve names for full moons, one for each month, and the name blue moon was used in years which had 13 full moons."

But in 1943, Sky and Telescope Magazine erroneously wrote that the second full moon in any calendar month was called a blue moon. The label stuck and is still used today.
Even more rare – there were two blue moons in 1999, one in January and one in March. That happens only about four times a century.