(...) When you hear someone say .Once in a blue moon.... you know what they mean: Rare. Seldom.
Maybe even absurd. After all, when was the last time you saw the moon turn blue?
According to modern folklore, a blue moon is the second full moon in a calendar month. Usually months have only one full moon, but occasionally a second one sneaks in. Full moons are separated by 29 days, while most months are 30 or 31 days long; so it is possible to fit two full moons in a single month. This happens every two and a half years, on average. (...)
source: UFMG/2006
WHEN IS THE NEXT BLUE MOON?