A Blue Moon This Way Comes
"Once in a blue moon" is an expression you may have heard before. It is now taken to mean "rarely", deriving from an 1821 writing, Real Life in London by Pierce Egan, where he writes:
"How’s Harry and Ben? - haven’t seen you this blue moon."
A celestial "blue moon" is now popularly known as the second full moon in a calendar month, and it in fact doesn't happen that rarely; about once every 2.5 years.