once in a blue moon

== English == === Etymology === See blue moon. First use appears c. 1833. See citation below. === Pronunciation === IPA(key): /wʌns ɪn ə bluː muːn/ === Adverb === once in a blue moon (not comparable) (idiomatic) Very rarely; very infrequently. Synonyms: once in a purple moon; see also Thesaurus:rarely ==== Translations ==== ==== See also ====