blue moon

American  

[bloo moon] / ˈblu ˈmun /

noun

1. a second full moon in a month, occurring every two to three years as a result of the discrepancy between a lunar year and a solar year.

2. (formerly) the third full moon in a season with four full moons instead of the usual three.

3. a long, indeterminate stretch of time, as between occurrences of a rare event or phenomenon.

4. an uncommon phenomenon in which the moon appears to have a blue tinge because of the filtering effect of smoke or dust particles in the atmosphere.

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idiom

1. a very long period of time.

   Such a chance comes once in a blue moon.

blue moon British  

noun

1. the second full moon occurring within a calendar month

2. informal very rarely; almost never

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Etymology

Origin of blue moon

First recorded in 1815–25

Example Sentences

Wright: Every blue moon, they would be in a good mood.

From Los Angeles Times • Apr. 16, 2025

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

From Salon • Mar. 21, 2025

So the second full moon in a month becomes the blue moon.

From BBC • Aug. 19, 2024

The full moon, which will be both a supermoon and a blue moon, will grace the skies Monday and will appear full until early Wednesday morning, according to NASA.

From Los Angeles Times • Aug. 19, 2024

Oh, I get it—it’s rare, like once in a blue moon.

From "Booked" by Kwame Alexander