once in a blue moon
WUNS in a BLOO MOON
/wʌns ɪn ə bluː muːn/
Very rarely; almost never.
A whimsical way to describe an infrequent occurrence.
The story behind the word. A 'blue moon' is the second full moon in a single calendar month, an event that happens roughly every 2.7 years. In the 16th century, the phrase meant something that was impossible, but it shifted to meaning 'rare' by the 19th century.
Word relationships
- seldom: 'Seldom' is formal; 'once in a blue moon' is more colorful and conversational.
- frequently: 'Frequently' means happening often.
Commonly confused with
- hardly ever: 'Hardly ever' is a direct synonym, but 'once in a blue moon' sounds more poetic.
Collocations
- happens once in a blue moon
- only once in a blue moon
- seen once in a blue moon
Example sentences
- "I only eat fast food once in a blue moon, usually when I'm on a long road trip." Personal habits
- "My sister lives in Australia, so we only get to see her once in a blue moon." Social life
Memory hook
Imagine looking at the sky and seeing a moon that is actually blue—it almost never happens! The rare blue light in the sky.
When not to use
Do not use for things that happen regularly, even if they are infrequent (like an annual birthday).
Fun facts
- The next blue moon will occur on May 31, 2026.
Related words
Explore more words
occasional, infrequent, seldom, whimsical, capricious, fluke
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