What is a blue moon?

“Once in a blue moon” is an expression for something that rarely happens.
Now, what is this blue moon?
A literal blue moon occurs when enough smoke or dust is present in the atmosphere to make the moon appear blue.
This occurred back in 1883 when the Indonesian volcano Krakatoa exploded,  and in  1950 when masses of smoke from Canadian bushfires filled the air.
Generally, the second full moon of a month is referred to as the blue moon. August 2012 had two full moons, August 2nd & 31st. The second blue moon isn't due until July 2015.