once in a blue moon
- 1Very rarely; only on unusually infrequent occasions.
Like a comet that swings past Earth just once in a lifetime.
Once in a blue moon, she treats herself to an expensive dinner downtown.
The old theater hosts a live performance only once in a blue moon these days.
Similarrarely, seldom, infrequently, scarcely, uncommonly
Oppositeoften, frequently, regularly, constantly
Note
An idiom of rarity that takes its picture from the sky, where a blue moon is itself an uncommon sight. The Athenæum was already using it in 1833 to ration foreign operas. The phrase is fixed: it takes no "very" or "more." It pairs naturally with only, placed before it or, as in James Hilton's Lost Horizon (1933), after it: "once in a blue moon only."
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Origin
The phrase derives from "blue moon" and first appears in use around 1833.