In the Hebrew calendar, based on a lunar cycle, there can never be two full moons in a month, by definition. The full moon of August 1st fell on the 15th of Av; the full moon of August 31st falls on the 14th of Elul. So, a blue moon is really just a coincidental occurrence. It happens 7 times in 19 years, a Metonic cycle of the moon, so about once every 2.7 years. Not exactly rare, but rare enough for the term "once in a blue moon" to have entered the common parlance, meaning an event that does not occur often. The next one is on July 31, 2015.
|"Blue Moon" rising over Machtesh Ramon.|
As it happened, the name came to be applied to any second full moon falling in the same calendrical month of the year. In the case of the Hebrew calendar this is the 15th of Elul, two weeks before Rosh Hashannah. The theme of Elul is "Hamelech Ba'Sadeh", The King is in the Field, accessible to all who wish to reach out to him, make amends and a better start for the new year about to begin. Kesivah and Chasimah Toavah to all k'lal Yisroel!