"Once in a blue moon"
I have always thought the same that IvyNeko and nealesa have written about this node, however, I have a meteorology textbook that offers a different explanation than theirs. I find it interesting and would like to share:
The scattering of light by large quantities of atmospheric particles can cause some rather unusual sights. If the volcanic ash, dust, smoke particles, or pollutants are roughly uniform in size, they can selectively scatter the sun's rays. Even at noon, various colored suns have appeared: orange suns, green suns, and even blue suns. For blue suns to appear, the size of the suspended particles must be similar to the wavelength of visible light. (This situation produces a type of scattering called Mie scattering.) When these particles are present they tend to scatter red light more than blue, which causes a bluing of the sun and a reddening of the sky. Although rare, the same phenomenon can happen to moonlight, making the moon appear blue; thus, the expression "once in a blue moon."
Source: Ahrens, Donald C. Meteorology Today: An Introduction to Weather, Climate, and the Environment. Pacific Grove, CA: Brooks/Cole, 2000. Page 86.
Please also take a look at The Cow's blue moon writeup for more insight.