A blue moon is the second full moon in a calendar month. It takes about 29 to 30 days for the moon to go from full back to full, or complete one cycle.
This typically only happens once in any given month, but about every 2.5 years, it happens twice in a month.
The moon doesn't actually look blue, but the saying "once in a blue moon" represents something that happens rarely.
The next blue moon is set for August 31, 2012.
For more information on a blue moon, check out the Farmer's Almanac at: http://www.farmersalmanac.com/what-is-a-blue-moon>