Some pedants maintain that a blue moon is the second full moon of a calendar month—a rather rare occurrence. While this is certainly one of the meanings, the original meaning is more general, referring to any rare occurrence.
The original sense of blue moon is that of an absurd event that can never occur. The moon is never really blue and once in a blue moon is akin to when pigs fly. (Well actually, when a lot of dust is kicked up into the atmosphere, the moon can appear blue. The eruption of Krakatoa in 1883 caused the moon to turn blue, as did late Indian monsoons in 1927 and Canadian forest fires in 1951.)
Copyright 1997-2015, by David Wilton