Last Saturday was a full moon, which looked like an ordinary full moon, but actually was a bit extraordinary a blue moon. A blue moon is the second full moon in a calendar month. For a blue moon to occur, the first of the full moons must appear at or near the beginning of the month so that the second will fall within the same month (the average span between two moons is 29.5 days). July 2004 had two full moons: the first on July 2, the second on July 31 that second full moon is called the blue moon. Obliquity.com has a great article with FAQs about the blue moon.