Once in a blue moon
Meaning
It describes an event that occurs very rarely or infrequently.
Origin
The phrase "once in a blue moon" derives its evocative power from a rare celestial event. A "blue moon" does not describe the moon's actual color, but rather the second full moon to appear within a single calendar month—a cosmic alignment that graces our skies only once every two or three years. This infrequent lunar phenomenon lent its precise rarity to the English lexicon, becoming the perfect, poetic shorthand for anything that happens only on the rarest of occasions. Though earlier usages of "blue moon" sometimes implied absurdity, it was the observable, albeit uncommon, astronomical occurrence that truly grounded the idiom in its modern meaning of extreme infrequency.
Examples
- My brother lives in Australia, so I only see him once in a blue moon.
- She used to visit the old bookshop often, but now she goes once in a blue moon.