artists:  Cowboy Junkies

songwriters:  Richard Rodgers and Lorenz Hart in 1934 for the film, "Hollywood Party"

release dates:  1954 by Elvis Presley; 1988 by the Cowboy Junkies

Rodgers and Hart wrote the lyrics to Blue Moon for actress, Jean Harlow, in a scene where she, as an innocent young girl, prays for stardom in the movies.  Countless artists have covered the song over time.

 Wikipedia definition of a "blue moon":  

The third full moon in a season of four full moons, or the second full moon in a calendar month.  The term, 'blue moon' comes from folklore, where different cultures and conventions place the 'blue moon' at different times of the year.

"Once in a blue moon" refers colloquially to mean "a rare event."