A blue moon is a full moon that is not timed to the regular monthly pattern. Most years have twelve full moons which occur approximately monthly, but in addition to those twelve full lunar cycles each calendar year contains an excess of roughly eleven days. The extra days accumulate, so that every two or three years (on average about every 2.7154 years) there is an extra full moon. The extra moon is called a "blue moon."
The term "blue moon" is commonly used describe the rarity of an event, as in the saying "once in a blue moon."
The next time there are two full moons in one month is December 2 and 31, 2009.
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