A blue moon is the appearance of the third full moon in a season that has four full moons, instead of the usual three. It is never visually blue. 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"


One lunation (an average lunar cycle) is 29.53 days. There are 365.26 days in a solar year. Therefore, about 12.37 lunations (365.26 days divided by 29.53 days) occur in a solar year. In the widely used Gregorian calendar, there are 12 months (the word month is derived from moon) in a year. Each calendar year contains roughly 11 days more than the number of days in 12 lunar cycles. The extra days accumulate, so every two or three years (7 times in the 19-year Metonic cycle), there is an extra full moon. The extra moon necessarily falls in one of the four seasons, giving that season four full moons instead of the usual three, and, hence, a blue moon.


http://en.wikipedia.org/wiki/Blue_moon