Glossary Home Idioms

Once in a blue moon

“Once in a blue moon” is a way of describing and emphasizing something, positive or negative, that happens very rarely.

“Once in a blue moon” is a commonly used English idiom that refers to the period of time between which two things happen. It dates back, in its current form, to at least the 1800s and perhaps earlier. Today, its believed to be connected to the occurrence of a second full moon in one month, referred to as a “blue moon.”

Once in a blue moon meaning



“Once in a blue moon” refers to something that happens very rarely. If something only occurs “once in a blue moon,” it happens once after a long time. It could be something that is positive or negative. The person who uses the phrase might’ve been looking forward to the event or dreading it. 

The exact period of time “once in a blue moon” refers to is not defined. It could refer to a week, a month, several years, or more. It might also be used hyperbolically in order to emphasize the time between two events when the period of separation is not really that long. 


When To Use “Once in a blue moon” 

It’s possible to use “once in a blue moon” in a wide variety of situations. It can be used when one person is describing another’s actions, an event, or a feeling. One might also use it to describe how they are themselves feeling or acting. Depending on the intonation a speaker uses, it’s fairly easy to determine whether they’re using the phrase excitedly to announce that something they want to have happen is finally occurring or if it’s something they’ve been dreading. 

For example, one friend might use the phrase to describe to another how often they see one another. Or, a mother might use the phrase to emphasize for her son how often they get to spend time together. 


Example Sentences

  • What have you been doing? We see each other once in a blue moon. 
  • I can’t believe what just happened, this can’t happen more than once in a blue moon. 
  • This was really a once in a blue moon event. 
  • Do you think you’ll ever have more time for me than a few hours once in a blue moon? 


Why Do Writers Use “Once in a blue moon?” 

Writers use “once in a blue moon” in the same way and for the same reasons that the phrase is used in everyday conversations. It’s possible to work the line into the dialogue between two characters or have a narrator use it in their narration of a particular scene or event. Depending on how an idiom such as this one is used, a reader may be more or less interested in the fact that the author used it. 

This phrase is quite well known and easily understood by most of the English-speaking world. Therefore, it’s unlikely that someone is going to come upon it in a piece of writing and not know what the author is referring to. But, it’s not impossible. It’s also easy to imagine a writer taking the premise of this phrase and altering it, using something else rather than a “blue moon” to emphasize time. 



“Once in a blue moon” is related to a celestial event—the appearance of two full moons in the same month. The second full moon is colloquially referred to as a “blue moon.” Whenever this happens, it occurs 32 months after the previous blue moon.

The first iteration of the phrase in print is believed to be from 1528 in a booklet printed by William Roy and Jeremy Barlowe. According to Idioms, the phrase read: 

O churche men are wyly foxes […] Yf they say the mone is blewe 

We must beleve that it is true 

Admittynge their interpretacion

There is another easier to understand and more contemporary version of the phrase that dates to 1821. This is believed to be the first written version of the phrase as it is used today. The line comes from Real Life in London by Pierce Egan. It reads: 

How’s Harry and Ben? – haven’t seen you this blue moon

As is often the case, the phrase is believed to have been popularized in the 1800s and has lasted until today. 


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