The full moon occurs at 8:45 p.m. on August 20th. It's called a Blue Moon but the name has nothing to do with color. The moon rises tonight at 7:47 p.m.
Occasionally, the full moon can take on a reddish color or can appear blue in color if a forest fire or volcanic eruption litters the upper atmosphere with ash or smoke. A volcanic eruption gave the moon a bluish tint from the perspective of many people on Earth in 1991 according to space.com.
The term Blue Moon was originally assigned to the third full moon in a four moon season, but after a mix up in "Sky & Telescope" magazine in 1943, the term took on a different meaning. An article in the magazine incorrectly defined a blue moon as the second full moon in a single month, but the definition stuck. This particular blue moon is a technical Blue Moon, but does not meet the criteria for the second definition. The third full moon in a four-full-moon season rule was detailed in a 1937 edition of the now defunct Maine Farmers' Almanac.
Tonight's full moon is also known as the Full Sturgeon Moon, the Green Corn Moon, the Grain Moon and the Full Red Moon. Each of August full moon names come from monikers given to the full moons of a certain month according to Native American and European traditions.
Blue Moons occur somewhat infrequently. After tonight's full moon, another Blue Moon will not occur again until 2015.