Wookiee
Two full moons in one month then?
From the little I've read about the subject, the two full moons in one month is a more recent definition.
The older traditional definition, which is recorded in early issues of the
Maine Farmer's Almanac, states that the blue moon is the third full moon in a season that has four full moons. The reasons for this is a bit more complex and it's derived from the Christian ecclesiastical calendar.