Folklore calls it a Blue Moon when it is the second full moon to appear in one month; the first occurred on August first. Our word for month is derived from the word moon.
The blue moon phenomena usually happens every couple of years. Depending upon what source you use, the next blue moon should fall on July 31, 2015 [Washington Post], or on August 21, 2013 [EarthSky News a Clear Voice for Science], etc.
While the moon is not actually blue in color, it has fostered several colorful customs – proposing under a blue moon, naming alcoholic beverages for it such as the Blue Moon Martini. And, in Idaho, there is a Blue Moon Café, Blue Moon Bar & Grill at Lava Hot Springs, and a Blue Moon Outfitters.
The name seems to have been derived from a goof-up when, in 1946, amateur astronomer James Hugh Pruett (Sky & Telescope) “incorrectly assumed how [the term] blue moon had been used in the Maine Farmers’ Almanac. Pruett unintentionally mangled the original blue moon definition, and it thus became the second full moon in a given month.”
According to an article from Mail Online, Science & Tech, blue moons came about after the eruption of the volcano Krakatoa in Indonesia in 1883, when “Ash soared right into the upper echelons of the atmosphere; blue moons were reported around the world, for up to two years.”
Americans took to the blue moon idea, and the concept lives on. The Slooh Space Camera broadcasted the lunar event this year and dedicated it to the late Neil Armstrong, America’s legendary astronaut who first set foot on the moon. Cosmically, the blue moon sighting fell on the day Armstrong was laid to rest. His family suggested looking up at the moon in the night sky and giving Neil a wink.