Once in a Blue Moon and Other Idioms That Don’t Make Scientific Sense – Smithsonian (blog)

Once in a Blue Moon and Other Idioms That Don't Make Scientific Sense
Smithsonian (blog)
Typically, 12 full moons occur from winter solstice to the next winter solstice (roughly three per season), but occasionally a forth full moon in a season could be observed. In such a case, one of the four full moons in that season was labeled “blue