Blue Moon
So, there is nothing blue about the Moon except the part in the expression, “once in a blue moon”.
Blue Moon on August 31, 2012, 9:40pm EST
Why Blue Moon?
Blue moon occurs every two or three years. Usually season has three full moons. When, on a rear occasions, or once in a blue moon, there are four moons in a season, the third appearance of the full moon is called Blue Moon.