But the meaning of a "blue moon" has changed over time. It originally meant, according to folklore, something more ludicrous than rare. According to a 2012 Sky & Telescope magazine article: "[The] very earliest uses of the term were remarkably like saying the Moon is made of green cheese. Both were obvious absurdities, about which there could be no doubt. 'He would argue the Moon was blue' was taken by the average person of the 16th century as we take 'He'd argue that black is white.' "
But a secondary definition — the one about two full moons in a month — popped up (as an error, it turned out) in a 1946 issue of Sky & Telescope. Years later, in 1980, the mistake was amplified by the public radio program StarDate, and later, by the board game Trivial Pursuit. It stuck.
Philip Hiscock, a folklorist at Memorial University in Newfoundland, and Texas astronomer Donald W. Olson "helped the magazine sort all this out and admit the mistake back in 1999. The error led to the widely accepted definition of blue moon today: the second full moon in a given month. A blue moon occurs roughly once every 2.7 years," according to Space.com.
Even so, "on rare occasions, the moon can turn [the color] blue," according to the NASA video. It says: "A truly blue moon usually requires a volcanic eruption. Back in 1883, for example, people saw blue moons almost every night after the Indonesian volcano Krakatoa exploded with a force of a 100-megaton nuclear bomb."