The saying "once in a blue moon" comes from the fact that a blue moon rarely happens. The common definition of a "blue moon" is two full moons that occur in a single calendar month.
When Meg told me that tonight's full moon would be a "blue moon," I didn't believe her. I wondered, "how can you have two full moons in the same month when one happens on the 20th?"
Well, I found that there is also a seasonal definition of a blue moon...the third of four full moons within a season...which tonight's will be! ~Murphy
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