Once in a blue moon...
Actually, our patrons ask us about blue moons far more often than "once in a blue moon!"
A "blue moon" has come to denote the second full moon within the same month, but only because of an error made in a 1946 article in the magazine "Sky and Telescope". Previous to this, the rule was that when a season contained four full moons, the third was known as a "blue moon". To read more about this rule, and the history of the error that caused this change in the definition of a blue moon, you can read "Sky and Telescope’s" "What’s a Blue Moon?" article from May 1999. (http://www.skyandtelescope.com/observing/objects/moon/3304131.html)
"Once in a Blue Moon" is a phrase that refers to something that in all likelihood will never happen at all or only far off in the future. It’s origin, though, precedes when we started to use the "blue moon" as a term for second full moon in the same month, or the third of four moons in one season. The expression itself derives from the extremely rare, actually blue-tinged Moon caused by atmospheric layers of forest-fire smoke or volcanic dust at just the right height and visual angle, a rare occurrence. And the term "blue moon" itself is actually well over 400 years old! For more of its history, you can read "Sky and Telescope’s" "Once in a Blue Moon" article from March 1999. (http://www.skyandtelescope.com/observing/objects/moon/3305141.html)
For more information on blue moons, this Once in a Blue Moon web site has a great FAQ list and a Blue Moon Calculator you can use to find out about upcoming blue moons. (http://www.obliquity.com/astro/bluemoon.html)