Once in a blue moon
/wʌns ɪn ə bluː muːn/informal, neutralUK · US
What does “Once in a blue moon” mean?
Once in a blue moon means very rarely.
The expression describes low frequency, not impossibility. It typically modifies a verb: we meet, he calls, it rains once in a blue moon.
It is mild and slightly wistful, and is acceptable in most everyday writing.
When do people say it?
Used about something that happens only at long intervals.
“We only see each other once in a blue moon these days.”
Is it formal or informal?
Register: informal, neutral. It is most at home in both everyday conversation and professional writing. Most common in UK and US.
Where does it come from?
Built on the rare astronomical 'blue moon'; the frequency sense is nineteenth century.
Origin confidence: Well documented
How do you say “Once in a blue moon” in other languages?
Related idioms
- Miss the boat, To be too late for an opportunity.
Last reviewed 2026-09-11