We have all heard the expression, "once in a blue moon," but now we will get a chance to witness the event on August 20-21, 2013, when the next blue moon is set to occur.
The phrase doesn't actually refer to the color of the moon, but instead the second of two full moons in a single calendar month or the third or fourth full moon in a season.
According to EarthSky "It’ll be a Blue Moon by the seasonal definition, that is, the third of four full moons to take place in a season, in this case between the June 2013 solstice and September equinox. The last Blue Moon by this definition happened on November 21, 2010."
