Have you ever heard the phrase "'once in a Blue Moon"? It means that something happens very rarely or almost never. In fact, this term is derived from astronomy, where it describes quite a curious and unusual phenomenon - an extra moon.

A blue moon is an additional full moon that appears in a subdivision of a year: either the third of four full moons in a season, or a second full moon in a month of the common calendar. This happens every two to three years (seven times in the Metonic cycle of 19 years). Extra moons are not colored blue. Actually, a moon can look blue, but such cases are extremely rare. Our natural satellite can appear blue when the atmosphere of the Earth is filled with dust or smoke particles of a particular size; slightly wider than 0.7 micron. Such a rare condition can be a consequence of a dust storm, a forest fire, or a volcanic eruption.

More Info: en.wikipedia.org