“Once in a blue moon” means very rarely, but the phrase’s history is less tidy than its familiar image of an unusual lunar event. The idea of a blue moon as something impossible or absurd appears in English by the 16th century. A satirical text from 1528, for example, invoked the moon being blue to suggest a claim no one should take literally. That early image helps explain the phrase’s roots, but it is not the same as the modern expression.
The wording “once in a blue moon” is recorded by the early 19th century, when it meant an occurrence so rare as to seem almost beyond belief. The expression likely grew from the older joke about an impossible blue moon, shifting from “never” toward “hardly ever.” The surviving evidence does not establish one precise moment or person responsible for that change, so the phrase is best understood as an evolution rather than a neatly traceable invention.
The calendar definition many people know—a second full moon in one month—came much later. Another, older almanac usage called the third full moon in a season with four full moons a blue moon. In 1946, an astronomy magazine described the term as referring to the second full moon in a calendar month, apparently misreading the almanac’s system. That simpler explanation caught on, and it is now widely used. Neither definition explains the original idiom directly.
A moon can look blue under unusual atmospheric conditions, such as when particles from fires or volcanic eruptions scatter light. But a calendar blue moon does not usually appear blue, and there is no good evidence that the idiom began with these rare optical events. “We only get together once in a blue moon now that we live in different states.” “The little bookstore hosts author events once in a blue moon, so I try not to miss them.”
The phrase endures because it turns rarity into a vivid, slightly whimsical picture—one that still makes sense even when the moon looks perfectly ordinary.