What Is 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 will have two full moons: the first on July 2, the second on July 31—that second full moon is called the Blue Moon. 
Two full moons in one month may occur in any month out of the year except for February, which is shorter than the lunar cycle. The last blue moon occurred on Nov. 30, 2001. 

An older definition for the blue moon is recorded in early issues of the Maine Farmer's Almanac. According to this definition, the blue moon is the third full moon in a season that has four full moons. 

But will it really be blue? Probably not. The date of a full moon, all by itself, doesn't affect the moon's color. The moon on July 31st will be pearly-gray, as usual.