Region: UK / US | Usage example: We only all get together once in a blue moon, so we cherish it.

Once in a blue moon

Meaning

Very rarely.

Used when emphasizing infrequency in both formal and informal speech.

Origin notes can vary by region and source; this page gives the commonly reported explanation and usage context.

Derived from the rare second full moon in a calendar month, dubbed 'blue' despite no color change.

Popularized in 19th-century British publications and later crossing into American idioms.