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A blue moon isn't actually blue but is actually a full moon that's the 2nd to appear within a calendar month. The lunar cycle is 29.5 days and a month is between 28 and 31 days so the probability of a full lunar cycle occurring precisely during this timeframe is therefore not very high hence the idiom "once in a blue moon." Actual blue moons have been reported under special circumstances such as a volcanic eruption or a forest fire which release plumes of particles that rise into the atmosphere that scatter away red light and only allow blue light through the dust cloud.
Contributed by Sam Feldstone