Once in a blue moon: Meaning and Examples

Updated 2026-10-03 · Idiom Meanings editorial team

Quick answer: The phrase once in a blue moon means that something happens very rarely or almost never. It describes events that occur with such low frequency that you are unlikely to see them again soon. Use this idiom to emphasize how unusual a specific occurrence truly is.

What Does "once in a blue moon" Mean?

The idiom means that an event happens very infrequently. It signals that the occurrence is so rare that it stands out against the backdrop of normal, everyday life.

This phrase conveys a sense of surprise or special occasion. When you use it, you highlight that the moment is exceptional and not part of your regular routine.

How People Use It Today

Modern speakers use this expression in casual conversation to describe hobbies, visits, or habits that seldom happen. It fits well when talking about family gatherings, special treats, or unexpected weather events that break the normal pattern of daily life.

Example Sentences

I eat out at restaurants only once in a blue moon because I prefer cooking at home.
My brother visits the city once in a blue moon since he lives far away.
We go to the theater once in a blue moon to support local actors.
She wears her best dress only once in a blue moon for special celebrations.
They play board games together once in a blue moon during holiday weekends.
I see my old college friends once in a blue moon at reunions.
The store offers deep discounts once in a blue moon during clearance sales.
He calls his grandparents once in a blue moon when he is traveling.
We drive to the beach once in a blue moon when the weather is perfect.
She dances in public once in a blue moon at community festivals.

Idioms With a Similar Meaning

Once in a while: This suggests infrequency but implies the event happens more often than once in a blue moon.
Every dog has its day: This focuses on a lucky moment rather than the rarity of the event itself.
A rare bird: This describes a person or thing that is unusual, not necessarily the timing of an event.
Now and then: This indicates occasional occurrence, which is less rare than the extreme infrequency of the main idiom.
Hardly ever: This is a direct statement of low frequency without the poetic imagery of a moon.
Once in a lifetime: This implies the event will likely never happen again, which is even rarer than the main phrase.

Opposite or Contrasting Phrases

Every day: This describes a routine event that happens without fail, the exact opposite of rarity.
All the time: This indicates constant or very frequent occurrence, contrasting sharply with infrequent events.
Regularly: This implies a predictable schedule, whereas the main idiom suggests unpredictability and scarcity.
Frequently: This means something happens often, which contradicts the core meaning of seldom occurring.

When to Use It and When Not To

Use this idiom in casual speech, emails to friends, or social media posts. It works well when you want to emphasize that an event is special because it is so uncommon and not part of your usual schedule.

Avoid using it in strict formal reports, legal documents, or academic papers. It is too colloquial for serious writing. Also, do not use it for events that happen regularly, as that would be a misuse of the phrase.

Example Dialogues

Are you coming to the potluck this Friday?

I would love to, but I only attend those events once in a blue moon.

Why do you have so many unopened magazines?

I subscribe to them out of habit, but I read them once in a blue moon.

Do you miss playing in the school band?

Not really. I only play an instrument once in a blue moon now that I work full time.

Is this seat taken during the conference?

No, go ahead. I only sit here once in a blue moon when I am feeling particularly energetic.

You look surprisingly clean shaven today.

Thanks! I usually wear a beard, but I shave once in a blue moon for professional photos.

More Example Sentences

The local library hosts a ghost story reading once in a blue moon, usually around Halloween.
He checks his email once in a blue moon because he prefers phone calls for urgent matters.
Snowfall in the desert occurs once in a blue moon and draws crowds from neighboring states.
She wears her silk evening gown once in a blue moon, saving it for truly memorable galas.
The office team retreat happens once in a blue moon, so everyone tries to make it count.
My father cooks a complex meal once in a blue moon, preferring simple dishes most nights.
A total solar eclipse passes over my city once in a blue moon, so I set multiple alarms.
He writes handwritten letters once in a blue moon, finding the process meditative and slow.

In Formal Writing, Say This Instead

Infrequently: The committee meets infrequently to review long-term strategic goals.
Rarely: Such anomalies in the data set are rarely encountered during standard testing.
Sparingly: The resource should be used sparingly to ensure sustainability for future projects.
With great rarity: These artifacts appear with great rarity in archaeological digs of this region.
On an occasional basis: Maintenance checks are performed on an occasional basis rather than daily.

Frequently Asked Questions

Is once in a blue moon a common expression?

Yes, it is a widely recognized idiom in English. People use it frequently in casual conversation to describe events that happen very rarely or with great infrequency in their daily lives.

Can you use this phrase in professional emails?

It is best reserved for casual or friendly professional emails. Avoid it in formal reports or official documents where precise language is required instead of colorful idioms.

What is the difference between this and once in a while?

Once in a blue moon implies something is extremely rare. Once in a while suggests something happens occasionally. The former is much less frequent than the latter phrase.

Does this idiom have a specific grammatical structure?

You can use it as an adverbial phrase at the beginning or end of a sentence. It modifies the verb to indicate the frequency of the action described.