EarthSky.org, 6/25/13, Deborah Byrd
There are different definitions for Blue Moon. By popular acclaim, the Blue Moon refers to the second of two full moons to occur in the same calendar month. A Blue Moon is also regarded as the third of four full moons in a single season – a season being defined as the time period between a solstice and an equinox, or vice versa. Or, someday, you might see an actual blue-colored moon. The next Blue Moon will fall on August 20-21, 2013. 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.
The next Blue Moon by the second-full-moon-in-a-calendar-month definition will be on July 31, 2015. The first full moon of July 2015 will be on July 1, 2015. Previously, the last monthly Blue Moon happened on August 31, 2012.
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