I found this information at the NASA site that you can find here
August 29, 2012: When someone says "Once in a Blue Moon," you know what they mean: Rare, seldom, even absurd.
This year it means August 31st.
For the second time this month, the Moon is about to become full. There was one full Moon on August 1st/2nd, and now a second is coming on August 31st. According to modern folklore, whenever there are two full Moons in a calendar month, the second one is "blue."
Don't forget to look at the Blue Moon tonight ... I'm sure it will be a beautiful site. Happiness all.