Blue moon

"Once in a blue moon" is a traditional phrase of unknown origin meaning "very rarely indeed". Webster's Dictionary (1949) defined a blue moon as meaning, "A phenomenon never, or very rarely, seen; hence, a rarely recurring occasion." However, since the mid-20th century the term blue moon has commonly been applied to the occurrence of a second full moon within a single calendar month, something that happens every two or three years, thus obscuring the original meaning of the term.

The use of blue moon to mean a specific calendrical event dates from 1937, when the Maine Farmers' Almanac gave a definition slightly different from the one now in common use, and was followed by a 1946 article in Sky and Telescope which led to the current usage, popular since the 1980s.

The phrase in modern usage has nothing to do with the actual color of the Moon, although a visually blue Moon (the Moon appearing with a bluish tinge) may occur under certain atmospheric conditions—for instance, if volcanic eruptions or fires release particles in the atmosphere of just the right size to preferentially scatter red light. This is a genuinely uncommon occurrence, more in accord with the original meaning of the phrase "once in a blue moon".