I've heard the phrase "once in a blue moon" used to mean "once in a great while". Looking it up on Wikipedia revealed that "blue moon" originally meant the third full moon in a season with four full moons, but that a popular mistake caused it to mean the second full moon in a calendar month (which is a definition I'd heard before).
But that didn't dispel all my curiosity: I've seen a "blue moon", and it's not any bluer than any normal full moon. So why is it called "blue"? Wikipedia becomes vague here:
The most literal meaning of blue moon is when the moon (not necessarily a full moon) appears to a casual observer to be unusually bluish, which is a rare event. The effect can be caused by smoke or dust particles in the atmosphere, as has happened after forest fires in Sweden and Canada in 1950 and 1951, and after the eruption of Krakatoa in 1883, which caused the moon to appear blue for nearly two years. Other less potent volcanos have also turned the moon blue.
...The key to a blue moon is having lots of particles slightly wider than the wavelength of red light (0.7 micrometre)--and no other sizes present. This is rare, but volcanoes sometimes produce such clouds, as do forest fires.
What does the color of the moon after forest fires or volcanoes have to do with its name when it happens to be the third full moon in a season with four full moons?