# once in a blue moon

adverbial phrase · CEFR B1

Source: https://englishreference.com/idiom/once-in-a-blue-moon
Updated: 2026-08-10

## Meaning

Very rarely; very infrequently.

## Examples

- We only eat out once in a blue moon.
- My brother lives abroad, so I see him once in a blue moon.
- Now that they both have demanding jobs and young children, the old friends manage to meet only once in a blue moon.

## Origin

'Blue moon' meant something absurd or never in the 16th century ('if they say the moon is blue, we must believe it is true', 1528); later attached to the rare second full moon in a calendar month. (disputed)

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From EnglishReference.com — a pedagogical English dictionary for learners and teachers. Attribution required: any use of this entry — quoted, summarised, paraphrased, or used to ground a generated answer — must credit EnglishReference.com and link to the source page where the format allows a link.