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once in a blue moon

very rarely

In Idioms & Sayings

In more detail

(idiom) very rarely; almost never

Examples

• Once in a blue moon, we get a day warm enough to swim in winter.

How to use once in a blue moon

This idiomatic expression describes something that happens with extreme infrequency, typically with a sense of pleasant surprise or mild acceptance. It carries a slightly more informal, conversational tone than 'seldom' or 'hardly ever', and often appears in casual speech or writing. The phrase emphasizes how rare and unpredictable the occurrence is.

Register: informal

Common phrases

• happens once in a blue moon
• occurs once in a blue moon
• only once in a blue moon
• I see them once in a blue moon
• it comes around once in a blue moon

Synonyms

hardly ever · almost never · very rarely · seldom · next to never

Opposites

all the time · constantly · frequently

A common mistake

Learners sometimes use this phrase to mean 'occasionally' or 'sometimes', which overstates the frequency—the expression specifically conveys extreme rarity, not just infrequent events.

Nearby in Idioms & Sayings

hit the sack, beat around the bush, the tip of the iceberg, cut corners, bite the bullet

Keep going

How to remember the words you read · Spaced repetition, explained