There are two full moons in July 2015. One occurs at 7:19 PM on July 1 and the other is at 3:43 AM on July 31. A second full moon during a month generally occurs every 2.7 years and is sometimes referred to as a ‘Blue Moon’. But in the original definition, a Blue Moon is not the second full moon of the month.
According to the Maine Farmer’s Almanac, the definition of a Blue Moon is the third of four full moons in a single season. This was mentioned in the 1937 issue of the almanac, which was the first known occurrence a Blue Moon was defined in print. The Blue Moon term associated with the second full moon of a month was first mentioned, possibly mistakenly, in 1946, but did not become popular until an astronomy-themed radio show mentioned this new association in 1980, according to SkyandTelescope.com.
This year, since summer began on June 21 and ends on September 23 with only three full moons on July 1st, July 31st, and August 29th, there is no Blue Moon.
Below is a SkyandTelescope.com illustration showing how rare a true Blue Moon is compared to a second full moon of a month.