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Blue Moon
A Blue Moon almost never looks blue — the name refers to timing, not color. There are two competing definitions still in use: the modern, more common one calls it the second Full Moon within a single calendar month (possible because a lunar month is about 29.5 days, slightly shorter than most months); the older, original definition — used by the Maine Farmers' Almanac — calls it the third Full Moon in a season that has four, instead of the usual three. Both versions are genuinely rare, happening roughly every two to three years, which is where "once in a Blue Moon" comes from as an idiom for something infrequent.
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What the Blue Moon means
The Moon can appear blue-toned in extremely rare cases — usually after large volcanic eruptions or wildfires fill the atmosphere with particles of a specific size that scatter red light — but that's a coincidence of timing, not the same phenomenon as a calendar Blue Moon.
