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." Different definitions place the "extra" moon at different times. The term "blue moon" is commonly used metaphorically to describe the rarity of an event, as in the idiomatic expression "once in a blue moon." The earliest recorded English usage of the term "blue moon" was in 1528 in a pamphlet violently attacking the English clergy, entitled Rede Me and Be Not Wrothe [Read me and be not angry]: "Yf they say the mone is belewe / We must believe that it is true" [If they say the moon is blue, we must believe that it is true].

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