once in a blue moon

See blue moon. First use appears c. 1833. See citation below.

• IPA^(key): /wʌns ɪn ə bluː muːn/

• Audio (General Australian):

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once in a blue moon (not comparable)

1. (idiomatic) Very rarely; very infrequently.

   Synonyms: see Thesaurus:rarely

   Coordinate term: once in a purple moon

   • 1833, J. Lection (publisher), The Athenæum, page 780:

     We are no advocates for the eternal system of producing foreign operas to the exclusion of the works of English composers, but once in a blue moon such a thing may be allowed.

   • 1933, James Hilton, Lost Horizon, published 1973, page 133:

     [T]ime expanded and space contracted, and the name Blue Moon took on a symbolic meaning, as if the future, so delicately plausible, were of a kind that might happen once in a blue moon only.