Almost everyone will tell you that a "blue moon" is the second full moon in a calendar month. But it hasn't always had that meaning. An earlier agricultural definition states that a blue moon is the third full moon in a season that has four full moons instead of the usual three. ABC News and Associated Press have written similar stories explaining the different meanings of the phrase. The About.com Astronomy site also has a page of links to facts, trivia, and legends about blue moons. Infoplease provides blue moon facts and myths, including an old proverb the Oxford English Dictionary says was first recorded in 1528:
If they say the moon is blue
We must believe that it is true.
What I found missing this year is the story that "Blue Moon" actually was the name of a Dutch trading vessel that returned to port so seldom that the folks named the intervals between port visits "Blue moons". Did I imagine it? Or does that story surface only once in a blue moon?
Just for fun, here's a list of Blue moon reading from the library's catalog - both fact and fiction.