Once in a blue moon Informal

To happen very rarely or infrequently, often used with a note of humor about just how uncommon the occurrence is.

It’s typically applied to pleasant surprises or exceptions to a normal pattern, rather than to disasters or emergencies.

The astronomical “blue moon” refers to an extra full moon appearing within a period that would usually contain only the expected number, and the modern popular definition — a second full moon occurring within a single calendar month — is actually the result of a documented mistake rather than genuine folk astronomy. The original 1937 definition, from the Maine Farmers’ Almanac, described a blue moon as the third full moon in a season that has four instead of the usual three; astronomer James Hugh Pruett misread this in a 1946 Sky & Telescope article and redefined it as the second full moon in a calendar month, an error later confirmed by researcher Donald W. Olson in 1999. Pruett’s simpler, easier-to-grasp version spread widely through later media, including a 1980 radio program and a 1986 trivia game, eventually becoming the standard popular understanding of “blue moon” despite not reflecting the original definition — making the modern astronomical usage itself a kind of accidental idiom shift. Either way, since an extra full moon of this kind happens only every two or three years, the phrase captures genuine rarity convincingly regardless of which definition is used.

He calls his parents once in a blue moon, usually just on birthdays.

We only eat out at fancy restaurants once in a blue moon.

Once in a blue moon, the whole family manages to be in the same city at once.

A: Do you still play tennis? B: Once in a blue moon, maybe twice a year.