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once in a blue moon

American  
[wuhns in uh bloo moon] / ˈwʌns ɪn ə ˈblu ˈmun /

idiom

  1. blue moon.

once in a blue moon Cultural  

  1. To do something “once in a blue moon” is to do it very rarely: “That company puts on a good performance only once in a blue moon.” The phrase refers to the appearance of a second full moon within a calendar month, which actually happens about every thirty-two months.

once in a blue moon More Idioms  

  1. Rarely, once in a very long time, as in We only see our daughter once in a blue moon. This term is something of a misnomer, because an actual blue moon—that is, the appearance of a second full moon in the same calendar month—occurs every 32 months or so. Further, the moon can appear blue in color at any time, depending on weather conditions. [Early 1800s]

Etymology

Origin of once in a blue moon

First recorded in 1830–40

Example Sentences

Examples are provided to illustrate real-world usage of words in context. Any opinions expressed do not reflect the views of Dictionary.com.

The Blue Moon is so-called because having two full Moons in a single month only happens once every few years, giving rise to the expression "once in a blue Moon."

From BBC

As it only happens every few years it is where the expression 'once in a blue moon' comes from.

From BBC

As it only happens once every few years, it is where we get the expression 'once in a blue Moon' from.

From BBC

“Once in a blue moon,” the frame reads.

From The Wall Street Journal

Once in a blue moon, a person is willing to do the hard internal work that leads to real, sustainable change.

From Salon

Definitions and idiom definitions from Dictionary.com Unabridged, based on the Random House Unabridged Dictionary, © Random House, Inc. 2023

Idioms from The American Heritage® Idioms Dictionary copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.