The article that told me I was wrong about what “blue moon” means (and hilariously refers to that definition as “trendy”) explains that the “two full moons in a month” definition is due to a misreading of the Maine Farmers’ Almanac: the real Blue Moon is the third full moon in a season that has four full moons. Why the third? The first full moon of the season is particularly significant; e.g., the Easter moon. The moons before and after have names as well – the Lent Moon precedes the Easter Moon, whether it’s the third or fourth moon of the winter – so the third moon of four is the extra one. By this definition, 2009 does not have a blue moon, since the full moon on December 31st is after the solstice and belongs to next year’s winter.