# What does ‘once in a blue moon’ mean?

To “once in a blue moon” means to very rarely. Brewer's records the astronomical and folk background: a 'blue moon' referred both to unusual atmospheric appearances (as after a volcanic eruption) and to a second full moon in a calendar month, both rare events.

## What it means

- Brewer's records the astronomical and folk background: a 'blue moon' referred both to unusual atmospheric appearances (as after a volcanic eruption) and to a second full moon in a calendar month, both rare events.

## Action steps

1. Common casual measure of infrequency.
2. Example: I only see my old classmates once in a blue moon.

## Sources

- [Brewer's Dictionary of Phrase and Fable (1898) — Internet Archive scan](https://archive.org/details/brewers-dictionary-of-phrase-and-fable)

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