A Blue Moon is an astronomical phenomenon. It's 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 solar calendar year contains an excess of roughly eleven days compared to the lunar year.
The extra days accumulate, so that every two or three years there is an extra full moon. And that's what's happening this year. 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.'
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