Once in a blue moon: meaning, examples and when to use it

Short answer: “Once in a blue moon” means not often at all. You use it for something that happens very rarely: “We eat out once in a blue moon.”

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What does “Once in a blue moon” mean in English?

A real blue moon is rare, so the idiom uses it for any event that almost never happens. It does not give an exact number. It can be once a year or even less.

Examples of “Once in a blue moon”

• My cousin visits once in a blue moon.
  A rare visit.
• I only eat candy once in a blue moon.
  A habit that you almost never do.
• It snows here once in a blue moon.
  An event that is very rare.

When do you use “Once in a blue moon”?

• To say that something happens very rarely.
• To describe a rare habit or a rare event.
• In speech and in writing.

Is “Once in a blue moon” formal?

“Once in a blue moon” is common and neutral. It is fine in speech and in most writing. In a very formal text, write “rarely”.

Common mistakes with “Once in a blue moon”

• It means rarely, not never.
• Say “once in a blue moon”, not “every blue moon”.
• Do not use it for things that happen often.

Similar expressions

• Compare: “very rarely” (plain English). It is in the group idioms about life, time and weather, with “under the weather”.

Questions

What does “Once in a blue moon” mean in English?

“Once in a blue moon” means not often at all. You use it for something that happens very rarely: “We eat out once in a blue moon.”

Is “Once in a blue moon” formal?

“Once in a blue moon” is common and neutral. It is fine in speech and in most writing. In a very formal text, write “rarely”.

How often is “once in a blue moon”?

There is no exact number. It means very rarely, maybe once a year or less.

Is it about the real moon?

No. The idiom just uses the rare blue moon as a picture for something rare.

Part of the English idioms guide and of idioms about everyday life, time and weather, in the 💬 Idioms tool. Đọc bằng tiếng Việt.

Written for English learners by the English for Fun team. How we write our lessons · Last updated 2026-11-05.