Idiom | Once in a Blue Moon

Idiom | Once in a Blue Moon

Idiom | Once in a Blue Moon

The idiom ‘once in a blue moon’ refers to an event that happens rarely, usually very rarely.

The idiom has nothing to do with the color of the moon, which can’t actually turn blue. Rather, it is about the frequency of a full moon in the same month. The phases of the moon are in 29.5 day cycles, so normally we see a full moon once every month. On some rare occasions, there will be two full moons in the same month. The second full moon is called the ‘blue’ moon.

Fun fact: A blue moon usually takes about 32 months to occur.   

The idiom dates back to the 1500s, but its form was modernized in the 1800s, making it a fairly old idiom. 

This idiom is very poetic in its phrasing, which probably has contributes to its popularity.



月は29.5日周期で動いていますので、1月に1回満月がみられるパターンが通常です。ただし、非常にまれではありますが、1月に2回満月になることがあります。この2回目の満月のことをBlue moonとよびます。

Blue moonは32ヶ月に1回通常でしたら起こる計算となります。



Idiom Usage | Once In A Blue Moon 使い方

English Stripped Bare - B. Bearington & Naomi - Idiom Once in a blue moon
A Full Moon Rising Over Downtown Chicago with Hancock Tower on the Right Side (

This idiom is quite easy to use. It’s only ever used in the positive form. It is never broken or split. It can be used either in the beginning or at the end of a sentence, making its placement flexible.  

Once in a blue moon is appropriate in both casual and formal situations. It is perfectly acceptable to use in business circumstances.







Examples 例文

English Stripped Bare - B. Bearington & Naomi - Idiom Once in a blue moon
A Champagne Cork Popping Off (

I only drink champagne once in a blue moon. Usually just on New Year’s Eve.


My boss is always traveling. She’s only in the office once in a blue moon.


I haven’t heard from my high school friends in a while. We only talk once in a blue moon.


Due to the pandemic, I don’t go out very much to restaurants, but once in a blue moon I’ll pick up some take-out.


Use mnemonic! ニーモニックテクニック

English Stripped Bare - B. Bearington Once in a blue moon
A blue moon? No, just Photoshopped (

You could imagine that seeing the moon turn blue would be a very rare event.

Of course, we always encourage you to create your own mnemonic device. It’s much more effect if you do that.

Check out our post on mnemonics basics to learn more.





Keep Studying

English Stripped Bare - B. Bearington & Naomi
English Stripped Bare Culture Page (G. McGregor/

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