In the first half of the 20th century a blue moon was referred to as the third full moon of a season — spring, summer, winter or fall, that has four full moons instead of the usual three. But that definition was misinterpreted over the years, and today we call the second full moon in a single month a "blue moon."
Because a blue moon occurs only every two or three years, the term blue moon is used colloquially to mean a rare event, as in the phrase "once in a blue moon." Blue moons come along once every 2.7 years on average, and sometimes much more frequently. The next blue moon will not occur again until July 2015.