According to Wikipedia a Blue Moon is :
” usually used to describe a second full moon in a single solar calendar month, which happens every two to three years (seven times in the Metonic cycle of 19 years). The usage of the term blue moon to describe the second full moon in one calendar month results from a misinterpretation of the traditional definition of that term in the March 1946 issue of Sky & Telescope. Beforehand, the term was used to describe the third full moon in a season with four. ”
Over the next 20 years there will 15 blue moons occurring, the last happening on 31st August 2012,
“In 1883, an Indonesian volcano named Krakatoa exploded. Scientists liken the blast to a 100-megaton nuclear bomb. Fully 600 km away, people heard the noise as loud as a cannon shot. Plumes of ash rose to the very top of Earth’s atmosphere. And the moon turned blue.
The term Blue Moon first came about in 1528 from a proverb recorded in the Oxford English Dictionary dictionary
“If the say the moon is blue
We must believe that it is true “