Blue moon
A blue moon is, in the modern popular sense, the second full moon in a single calendar month; in the older almanac sense, the third full moon in a season that has four. It happens because the 29.5-day lunar month is slightly shorter than most calendar months. The phrase once in a blue moon marks the same rarity, which returns roughly every two to three years.
Two definitions
The older one, from American farming almanacs, called a blue moon the third full moon in a season with four full moons. The one most common today, born from a simplified reading of a 1946 article, takes the second full moon within a single month. Both coexist: depending on the rule you pick, the same year can hold zero or one blue moon.
Why blue
The Moon is almost never blue: the color is a figure of speech. The word mostly means rarity, as in once in a blue moon. The Moon can occasionally look genuinely bluish, after a large volcanic eruption or a vast wildfire, when particles of a certain size filter out red light. It is rare, and unrelated to the calendar.
Sources
- , Sky & Telescope (1946)