Once in a blue moon means very rarely, almost never. People use it for things that hardly ever happen: seeing a relative who lives abroad, or a very busy person going to the theatre. It sounds informal and slightly playful. The idiom fits IELTS Speaking, but in an essay use rarely or very seldom instead.
When would you say once in a blue moon?
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(only) once in a blue moon; do something + (only) once in a blue moon
- My brother lives in Canada, so we only see each other once in a blue moon.
- I only eat fast food once in a blue moon, usually when I'm travelling.
- Our boss works from home once in a blue moon, so everyone noticed when she did it twice last week.
What changed
First versionI go to the cinema once in a hundred years.
RevisionI go to the cinema once in a blue moon.
Once in a hundred years is understood literally in English: once a century, as people say about a rare flood. It does not work as a playful way to say very rarely. It was replaced with once in a blue moon, and the rest of the sentence stayed the same.
Once in a blue moon means almost never. For something that happens now and then but is not that rare, use from time to time: I visit my old school from time to time. For meetings once a month, once in a blue moon would be an obvious exaggeration.
Your turn
My uncle lives on the other side of the world, so we see him only ___, maybe once every five years.
Show the answer
once in a blue moon — Once every five years is very rare, which is exactly once in a blue moon. Out of the blue means suddenly and unexpectedly, and it says nothing about how often something happens.
Write about an activity you enjoy but hardly ever have time for. Say what it is and why you do it so rarely.
I love skiing, but I go only once in a blue moon because the nearest resort is six hours away.
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