24/09/2026
We say this when something happens very rarely.
But here's the interesting part: the expression actually comes from astronomy.
A blue moon isn't usually a moon that looks blue. It's the name given to a second full moon that occurs within the same calendar month. Because full moons are about 29½ days apart, this doesn't happen often.
That rarity is exactly what gave the expression its meaning.
Examples:
We only see each other once in a blue moon.
He cooks once in a blue moon.
A phrase that started with the calendar became one of English's favourite ways to describe something that hardly ever happens.
Grammar Gate — exploring the stories behind the English we use every day.