Idiom
Once in a blue moon
By EasySay. Published . Updated .
Once in a blue moon means very rarely.
You use it for something that happens so seldom that each time feels special or surprising. It does not name a real date. People say it in friendly talk and in writing in Australia, the UK and the US.
Written for EasySay, in our own words.
In short
- AvoidDo not use it for something that happens every month or every year.
- Or sayHardly ever
Once in a blue moon: what it is, and what it is not
- IS
- Very rarely; something that happens only now and then.
- IS
- Longman labels it informal.
- IS
- About 0.07 times per million words in British books, 2010 to 2019.
- IS
- About 0.05 times per million words in American books, 2010 to 2019.
- IS NOT
- About a blue moon: it means very rarely, only now and then.
Meaning: Very rarely; something that happens only now and then.
How do people say it?
- In conversation:
We see my cousins once in a blue moon, because they live overseas.
- At work:
Our manager works from the office only once in a blue moon.
Check yourself
What should I know?
The words say
One time, when the moon looks blue.
It really means
Very rarely; only now and then.
What do dictionaries say?
- In Australia
- Longman labels it informal. The dictionaries we checked give it no Australian label, so it is general English, not only from Australia.
- In the UK
- Google Books (2010 to 2019): about 0.07 times per million words in British books.
- In the US
- Google Books (2010 to 2019): about 0.05 times per million words in American books.
What mistakes should I avoid?
- Do not use it for something that happens every month or every year. The idea is much rarer than that.
- Keep the words in order: “once in a blue moon”. People do not say “one time in a blue moon”.
- It is about how often, not about the real moon. You do not need to look at the sky.
The phrase most often sits at the end of a sentence, after the verb, and it answers the question “How often?” You can say “We meet once in a blue moon.”
You can also say “I cook once in a blue moon” or “It rains here once in a blue moon.” It also works at the start of a sentence for emphasis: “Once in a blue moon, you meet someone who really listens.” Keep the words and the order as they are.
Choose it when you want to be kind and a little playful about how rarely something happens. It is gentler than a flat “no”, because it leaves a small gap for the rare time.
At work you might say, “Our team works on Saturdays once in a blue moon.” That tells your listener that it happens, but not often. If the thing does not happen at all, say so plainly.
To remember it, think of how seldom people see something special in the night sky. You look up again and again, and the sky looks ordinary. Then, on a very rare night, something unusual appears.
The phrase takes that idea into everyday life. When you read it, ask yourself how many times a year the thing happens. The answer is usually close to none, and that is the point of the phrase.
What else can I say?
- Hardly ever Plain and common. People say “I hardly ever watch television.”
- Very rarely Plain and slightly more formal. People say “We very rarely meet.”
- Not very often The gentlest choice. It works in any place.
More idioms
- Burn the midnight oil To work or study late into the night, often to meet a deadline.
- A dime a dozen Very common and ordinary; easy to find and not special.
- When pigs fly It will not happen; said with a laugh about the impossible.
More examples
- In a text:
I eat fast food once in a blue moon. Today was one of those days.
Questions people ask
Is the moon really blue?
No. The phrase is not about the colour of the moon. It is only a picture for something that happens very, very rarely.
Can I say “once in a blue moon” in a job interview?
Yes. It is friendly and clear, for example “I work on weekends only once in a blue moon.” Plain words such as “rarely” are a safe alternative.
How rare is “once in a blue moon”?
It is very rare, much rarer than “sometimes” or “now and then”. It does not give an exact number. It names something that happens only now and then.
Sources and dates
- EasySay's own writing for the idioms, EasySay. Licence: original writing for EasySay. Checked .
- Longman Dictionary of Contemporary English: usage labels, Pearson Education. Licence: our own words, from the facts in the source. Checked .
- Google Books Ngram Viewer: counts per million words, 2010 to 2019, Google. Licence: our own words, from the facts in the source. Checked . Counts read from the Google Books Ngram Viewer (British and American English corpora, 2010 to 2019). Google publishes the Ngram data under CC BY 3.0 (https://creativecommons.org/licenses/by/3.0/).
Cite this page
EasySay. "Once in a blue moon". https://easysay.org/idioms/once-in-a-blue-moon. Last updated 23 October 2026.
https://easysay.org/idioms/once-in-a-blue-moon