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). May 2007 had two full moons: the first on May 2, the second on May 31—that second full moon was called the blue moon.

Some years have an extra full moon—thirteen instead of twelve.