"Once in a Blue Moon" is a common quip for a rare event -- but where does the phrase come from?
First off, much to everyone's surprise, the term has nothing to do with the color of the Moon. Rather it's said to be the second full Moon in a single calendar month. Full Moons occur every 29 days. So once every 2.7 years, two full Moons occur in the same month.
Originally, the term Blue Moon was reserved for Moons that had no other name. The Farmer's Almanac used traditional American colonial folk names for the 12 full Moons of the year, three Moons for each of the four seasons. But every few three years, a season will have four Moons instead of three. The third Moon was called the "Blue Moon," like a Moon name wildcard.
Neither a calendrical Blue Moon nor a seasonal Blue Moon has any particular astrological significance. Astrologers take note when two full Moons in a row fall in the same zodiacal sign, also about once every three years. That zodiac sign gets a double-dose of lunar energy (a gift each sign receives about once every 36 years).
The full Moon on August 20, 2013 is just such a Moon -- and the lucky winner is Aquarius! A full Moon in Aquarius emotionally charges this traditionally cool-headed sign. If Mr. Spock were an Aquarian, this would be the season of pon-farr mating madness. For us mere morals, we are challenged to allow space to experience our emotions without judgment. No matter what storms may rage within or around us, we're encouraged to remain aware yet detached, and resist being completely swept away.