What is a Blue Moon and when is the next one?
Because the time between two full Moons doesn't quite equal a whole month, approximately every three years there are two full Moons in one calendar month. Over the past few decades, the second full Moon has been come to be known as a "blue Moon." The next time two full Moons occur in the same month (as seen from the United States) will be December 2009. The most recent "blue Moon" occurred in May 2007.
On average, there's a Blue Moon about every 33 months. Blue Moons are rare because the Moon is full every 29 and a half days, so the timing has to be just right to squeeze two full Moons into a calendar month. The timing has to be really precise to fit two Blue Moons into a single year. It can only happen on either side of February, whose 28-day span is short enough time span to have NO full Moons during the month.
The term "blue Moon" has not always been used this way, however. While the exact origin of the phrase remains unclear, it does in fact refer to a rare blue coloring of the Moon caused by high-altitude dust particles. Most sources credit this unusual event, occurring only "once in a blue moon," as the true progenitor of the colorful phrase.
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