Blue Moons

A blue moon refers either to the presence of a second full moon in a calendar month, to the third full moon in a season containing four, or to a moon that appears blue due to atmospheric effects.
The calendrical meaning of "blue moon" is unconnected to the other meanings. It is often referred to as “traditional”, but since no occurrences are known prior to 1937 it is better described as an invented tradition or “modern American folklore”.... plus