Meaning of “once in a blue moon” in the English Dictionary

british dictionary

"once in a blue moon" in British English

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once in a blue moon

not very often:

My sister lives in Alaska, so I only see her once in a blue moon.

(Definition of “once in a blue moon” from the Cambridge Advanced Learner’s Dictionary & Thesaurus © Cambridge University Press)

"once in a blue moon" in American English

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once in a blue moon

rarely:

Once in a blue moon he’d call, but for months at a time we heard nothing.

(Definition of “once in a blue moon” from the Cambridge Academic Content Dictionary © Cambridge University Press)