How do you say “Once in a blue moon” in French?
The closest natural French equivalent is Tous les 36 du mois, which literally means “Every 36th of the month”. English speakers use once in a blue moon to mean very rarely. This is the most natural idiomatic equivalent for describing something that happens exceptionally rarely. Native speakers use it often in everyday spoken French. It is informal and common in everyday French.
Closest natural French
Tous les 36 du mois
/tu le tʁɑ̃t.sis dy mwa/
- Literal meaning
- “Every 36th of the month”
- Register
- informal
- How often it's used
- common
- Equivalence
Near-exact equivalent
90 out of 100
What a native French speaker would actually say
The English idiom "once in a blue moon" draws upon an astronomical oddity, an extra full moon in a season or calendar month, to represent something that happens genuinely, but very infrequently. In French, the parallel idea is conveyed not through real celestial cycles, but through humorous calendar absurdity: inventing a 36th day that does not exist in any month.
While "tous les 36 du mois" leans slightly toward hyperbole (flirting with the idea of "virtually never"), native speakers routinely use it in casual conversation to highlight extreme rarity just like "once in a blue moon." If you need to express the concept in a professional or written register, you should default to a straightforward adverbial phrase such as "très rarement."
How close is the meaning?
- Meaning5 out of 5
- Tone4 out of 5
- Context4 out of 5
- Literal similarity1 out of 5
These ratings are editorial approximations, not measurements.
What are other ways to say it in French?
Très rarement
Literally: “Very rarely”
Neutral and standard; suitable for all contexts and formal writing.
Une fois tous les tremblements de terre
Literally: “Once every earthquake”
A colloquial, slightly humorous variant meaning once in a very long while.
À la Saint-Glinglin
Literally: “On Saint-Glinglin's Day”
Means 'never' or 'when pigs fly', rather than just extremely rare.
Don't translate it literally
Une fois dans une lune bleue
This is an unidiomatic literal calque from English that makes no figurative sense in French.
Word-for-word versions like these are understood as literal statements in French, not as expressions.
In context
English
He only cleans his apartment once in a blue moon.
French
Il ne nettoie son appartement que tous les 36 du mois.
English
We only see each other once in a blue moon these days.
French
On ne se voit que tous les 36 du mois ces derniers temps.
Related expressions
- What “Once in a blue moon” means in English
- Miss the boat, To be too late for an opportunity.
Last reviewed 2026-09-11