Once In A Blue Moon – Meaning, Origin and Usage

We have heard the phrase “once in a blue moon,” but what does it mean? The saying is derived from the fact that blue moons are rare occurrences. That is why the phrase has come to mean something rarely happens. This post unpacks the meaning and origin of this expression.


The phrase “once in a blue moon” is used to explain something that happens seldom or rarely. There are two main meanings, the first is an actual blue moon. Just about every three years there is a full moon twice a month. When two full months occur within a single month, it is called a blue moon. Another meaning of blue moon is that there happen to be four complete full moon nights within a single season. There are typically only three full moons each season, which means that this is a semi-rare, but calculable occurrence.

The phrase “once in a blue moon” often describes how long something takes to happen. For example, you might say, “I only go out to eat once in a blue moon.” This means that you don’t go out to eat very often. While once in a blue moon may not reach the level of almost never, it does mean that it is something that does not occur with any regularity.

Example Usage

“I haven’t seen my old friends in years; it feels like once in a blue moon.”

“I know you’re busy, but can we meet up for lunch once in a blue moon?”

“I don’t usually drink coffee, but every once in a blue moon I’ll have a cup.”

“Once in a blue moon, I’ll make an exception and have dessert before dinner.”


James Hugh Pruett (1886-1955), an American writer and humorist, is credited with first popularizing the saying “once in a blue moon” in his 1946 article about Sky and Telescope. The saying is derived from the astronomical event known as a blue moon, which is quite rare. The phrase “blue moon” has been used since the 16th century to describe a rare occurrence.

Phrases Similar to Once In A Blue Moon

  • “As rare as hen’s teeth,” means exceptional or impossible to occur.
  • “In a million years,” means very unlikely to happen.
  • “Once in a lifetime,” means an event that is not likely to happen again.
  • “Once in a coon’s age,” means an event that is not likely to happen again.
  • “A needle in a haystack,” means something challenging to find.
  • “A drop in the bucket,” means a small and insignificant amount.

Phrases opposite to Once In A Blue Moon

  • “Once a week,” means frequent.
  • “Every other day,” means often.
  • “Twice a day,” means very common.

What is The Correct Saying?

  • The correct saying is “once in a blue moon.”

Ways People May Incorrectly Say Once In A Blue Moon

“Once in a blue moon” means something that happens very rarely. People often incorrectly say “once in a while” or “every now and then” to mean the same thing. Some examples are:

  • John comes by once in a blue moon, typically every Friday at noon.
  • I like to eat at the buffet once a month, which is really akin to once in a blue moon.

Acceptable Ways to Phrase Once In A Blue Moon

If you want to sound more educated, use the phrase “once in a blue moon.” It will make you seem more thoughtful and more well-read. Some more acceptable examples are:

  • I only eat cheesecake once in a blue moon because it has so many calories.
  • It is so nice to hear from you, it’s been a blue moon since you phoned me last!
  • Sorry, I won’t be able to make it, I only go to concerts once in a blue moon due to my work schedule.

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