Blue Moons

"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... meer