This year it means "the end of July." For the second time in July, 2015, the Moon is about to become full. There was one full Moon on July 2nd, and the second on July 31st. The above shot is one I took on that evening... photoshopped blue (LOL).
According to modern folklore, the second full Moon in a calendar month is "blue."
Strange but true: Sometimes the Moon really turns blue.
You can see a blue moon several ways. The image below was photographed on July 25th by Giuseppe Petricca of Pisa, Italy. In it he increased the saturation of the natural colors of the Moon.
The areas that emerge in a strong blue color (mostly in the Maria regions) are areas rich in Titanium, proof of ancient lava flows, and those contain more titanium in contrast to the Maria orange regions, that are poorer of this material.
A truly-blue Moon can be seen after a major a volcanic eruption. Back in 1883, for example, people saw blue moons almost every night after the Indonesian volcano Krakatoa exploded with the force of a 100-megaton nuclear bomb. Plumes of ash rose to the very top of Earth's atmosphere, and the Moon became an azure-colored disk.
Krakatoa's ash was the reason. Some of the plumes were filled with particles 1 micron wide, about the same as the wavelength of red light. Particles of this special size strongly scatter red light, while allowing blue light to pass through. Krakatoa's clouds thus acted like a blue filter. People also saw blue-colored Moons in 1983 after the eruption of the El Chichon volcano in Mexico. And there are reports of blue Moons caused by Mt. St. Helens in 1980 and Mount Pinatubo in 1991.
Forest fires can do the same trick. A famous example is the giant muskeg fire of Sept. 1953 in Alberta, Canada. Clouds of smoke containing micron-sized oil droplets produced lavender suns and blue Moons all the way from North America to England.
Comments or questions are always welcomed (well, as least nice ones)!