Originally “blue moon” referred to an absurdity according to the Oxford English Dictionary which notes a reference to a proverb from 1528 “If they say the moon is blue, we must believe that it is true”. However the moon can actually appear blue.
Although it happens only rarely when small particles in the atmosphere scatter more red light than blue allowing less red light to be seen from the ground the moon temporarily appears blue. Although this phenomenon is quite rare it has altered the meaning of the phrase “once in a blue moon” which now commonly is understood to mean “every now and then”.
The term also often refers to the second full moon in a given month. In an article published in the Sky & Telescope magazine in 1946 James H. Pruett used the phrase to refer to the second full moon to occur during a given month. While this was a misinterpretation of the original meaning for “blue moon” after a couple of decades of popular use (or should that be misuse?) we now also consider the 13th full moon in a year to be a “blue moon”.
The next blue moon, according to the Sky & Telescope convention, will occur in June 2007.