For one thing, a blue moon isn't blue! Originally, it meant something that would hardly ever happen, as in "We will have a blue moon, before such and so " Then it became the name for the third full moon in a season (three months) of four full moons Then about 50 years ago it became the name of the second full moon in a month, as Mar 31, 1999 They are not very rare They happen every two or three years