"Blue Moon" is a classic popular song. It was written by Richard Rodgers and Lorenz Hart in 1934, and has become a standard ballad. The lyrics presumably refer to an English idiomatic expression: "once in a blue moon" means very rarely.
A blue moon can refer to the third full moon in a season with four full moons, or the second full moon of a calendar month. Most years have twelve full moons which occur approximately monthly, but in addition to those twelve full lunar cycles, each solar calendar year contains an excess of roughly eleven days compared to the lunar year of 12 lunations.
Source:
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