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). May 2007 will have two full moons: the first on May 2, the second on May 31—that second full moon is called the blue moon.
Note that the May 31 date applies to most of the Western Hemisphere, including the United States. In the Eastern Hemisphere, the full moon in question will occur on June 1. For that half of the world, the blue moon will be on June 30, 2007.
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