A blue moon is an extra full moon that appears in a subdivision of a year, either the third of four full moons in a season or, recently, a second full moon in a month of the common calendar. Metaphorically, a "blue moon" is a rare event, as in the expression "once in a blue moon".
The phrase has nothing to do with the actual color of the moon, although a literal "blue moon" (the moon appearing with a tinge of blue) may occur in certain atmospheric conditions; e.g., when there are volcanic eruptions or when exceptionally large fires leave particles in the atmosphere.
Fun fact: Did you know that Twice In A Blue Moon was the title track of the third album of Ferry Corsten?
Requested for the Corsten's Countdown 2013 NYE special by: Manizha, Spiridon SirbuPlayed in: