Blue Moon is defined as the second full moon in a calendar month. The first full moon was August 1. The second full moon was on August 31, 2012. The second full moon is called Blue Moon. Because this occurance doesn't happen very often, thus the coined phrase, "Once in a Blue Moon"
I realize that the above photo is not the best quality (blurry and overexposed)... but it was the best I got with the camera I had.