“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.”
Read more at NPR