A blue moon happens when there are two full moons in the one month. Blue moons occur only 41 times in a century.
Edit: Hmm, just did a bit of research and now I am totally confused. According to Wiki the blue moon is the one at the end of June!
http://en.wikipedia.org/wiki/Blue_Moon
Edited by frantique, 01 June 2007 - 02:08 AM.