Once in a blue moon

Meaning

Very rarely.

Usage

Used when emphasizing infrequency in both formal and informal speech.

Origin & History

Origins and dates here are approximate summaries. Many idioms have disputed or unknown origins, and not every date has been checked against a historical dictionary, so treat specific dates and stories as a starting point rather than a citation.

From the older idea that a blue moon is something that almost never happens. The meaning of a second full moon in a month came from a 1946 magazine error.

'Blue moon' for something absurd or impossible dates from the 1500s; 'once in a blue moon' is recorded from the 19th century.