According to the Google Calculator, once in a blue moon is 1.16699016 × 10-8 hertz.
Folklore gave each moon a name according to its time of year. A moon that came too early had no folk name, and was called a blue moon, retaining the correct seasonal timings for future moons.
The Farmers' Almanac defined blue moon as an extra full moon that occurred in a season; one season was normally three full moons. If a season had four full moons, then the third full moon was named a blue moon.
The March 1946 Sky and Telescope article Once in a Blue Moon by James Hugh Pruett misinterpreted the 1937 Maine Farmers' Almanac. Seven times in 19 years there were — and still are — 13 full moons in a year. This gives 11 months with one full moon each and one with two. This second in a month was called Blue Moon. Widespread adoption of the definition of a blue moon as the second full moon in a month followed its use on the popular radio program StarDate on January 31, 1980.
Using the Farmers' Almanac definition of blue moon (meaning the third full moon in a season of four full moons), the next blue moon will not occur until May 21, 2016.
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