Once in a Blue Moon

I always wondered for a very long time what a blue moon was and tonight i thought I'd look it up to find out:
Wikipedia explains:

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.
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.