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‘Once in a blue moon’ is a situation or event that happens infrequently.
Examples:
“I only go to the cinema once in a blue moon.”
Or
“Once in a blue moon, I buy a fashion magazine, just to see what people are wearing.”
The Origins
In the early days the definition meant something that was impossible or would never happen, similar to the phrase “when pigs fly.” It wasn’t until 1821 with the publication of Real Life in London by Pierce Egan that we see “once in a blue moon” meaning “rarely.”