According to www.infoplease.com there are in fact two definitions for a blue moon. According to the more recent definition, 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). If you would like to know more about the Blue Moon phenomenon click here: Blue Moon Information