Once in a Blue Moon | Meaning, Examples & Origin

Once in a blue moon is an idiom that means “not very often” or “rarely.” It’s used to express that something doesn’t occur regularly. For example, if someone lives far away from the coast, it’s possible that they only go to the beach “once in a blue moon,” meaning they hardly, if ever, go.

Examples: Once in a blue moon in a sentence
Once in a blue moon, my sister and I go out dancing, but only when she’s in town.

I play the lottery once in a blue moon to try my luck.

My grandma, who prefers to stay at home, agrees to go on vacation with me once in a blue moon.

How to use once in a blue moon

“Once in a blue moon” is an idiom, which is an expression that conveys a meaning different from the literal definitions of the individual words that make it up. It is a casual and informal phrase that can be used in place of words and phrases like “hardly ever,” “rarely,” “seldom,” and “infrequently.”

“Once in a blue moon” can be found at the beginning, middle, or end of a sentence. When used at the beginning, a comma is required after it.

Examples: Once in a blue moon at the beginning of a sentence
  • Once in a blue moon, I go swimming in the lake behind my house.
  • Once in a blue moon I go swimming in the lake behind my house.

Some people add the word “every” before using the idiom “once in a blue moon.” Although both variations are acceptable, the use of “every” is not necessary, and some would say it’s superfluous.

Examples: Every once in a blue moon in a sentence
Acceptable: We live in Florida and go watch the shuttle launches every once in a blue moon.
Preferred: We live in Florida and go watch the shuttle launches once in a blue moon.

Once in a blue moon origin

While it’s not known exactly when and where the expression “once in a blue moon” originated, the term “blue moon” once referred to something that is extremely rare: a fourth full moon in a season, which typically has only three.

However, in 1937, the Maine Farmers’ Almanac provided a slightly different definition of the term “blue moon,” which is now the common use. Today, a “blue moon” refers to the second full moon in a month (which typically has only one). This event isn’t as rare, happening every few years.

In any case, “once in a blue moon” uses figurative language to express “rarely” or “not very often.”

Frequently asked questions about once in a blue moon

What’s another way of saying once in a blue moon?

Synonyms you can use in place of “once in a blue moon” include:

  • From time to time
  • Hardly ever
  • Not often
  • Rarely
  • Seldom
How often is once in a blue moon?

Once in a blue moon” is an idiom that means something happens “infrequently” or “rarely.” For instance, because it’s usually hot and sunny in Las Vegas, one could say that it only rains there once in a blue moon.

What is a blue moon?

In modern terms, a “blue moon” refers to the second full moon of the month, which typically has only one full moon. This happens once every few years.

However, historically, a “blue moon” referred to a rarer phenomenon: the fourth full moon in a season, which would typically have only three. That is why people use the idiomonce in a blue moon” to mean “very rarely.”

Is this article helpful?
Gina Rancaño, BA

Gina holds a Bachelor of Arts in English, as well as a certificate in professional and public writing from Florida International University. When she’s not writing, she spends her time reading.