This year, there will be a full moon on Halloween. This makes it a “blue moon,” since it’s the second full moon in one calendar month. This phenomenon usually happens around every 2.7 years. So maybe something that happens “once in a blue moon” isn’t actually that rare after all.
However, the blue moons we know and love — when two full moons can be seen in a month — is a comparatively recent definition.
As Corrine Rojas, a planetary scientist, explains that the term blue moon used to refer to the third of four full moons to happen in a season — specifically, the moon “between an equinox and a solstice in that year.” That’s because astronomers define seasons according to equinoxes and solstices, as opposed to the three-month chunks meteorologists use.
According to earthsky.org, this switch from the seasonal blue moon stems from a 1946 oversimplification in Sky and Telescope magazine. The mistake was apparently then amplified on the radio show StarDate and, later, by Trivial Pursuit.
In both cases, there’s a bonus moon.