'Once in a blue moon' meant something absurd long before it meant something rare, the original sense was closer to 'when pigs fly'.

A sixteenth-century pamphlet used it to mock believing an obvious impossibility. It drifted to mean 'very seldom', and then a 1946 magazine error attached it to the second full moon in a month, which is where it sits today. Three different meanings, none of them involving a blue moon.

Tradition: English idiom, sixteenth century onward · checked Aug 2026

🔭 Meanwhile, in astronomy: The blue moon rule is a 1946 mistake

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'Once in a blue moon' meant something absurd long before it meant something rare, the original sense was closer to 'when pigs fly'.

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'Once in a blue moon' meant something absurd long before it meant something rare, the original sense was closer to 'when pigs fly'.

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