There are different definitions for Blue Moon. By popular acclaim, Blue Moon refers to the second of two full moons in a single calendar month. A Blue Moon might also be 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.
The older definition, which is recorded in early issues of the Maine Farmer's Almanac, states that the blue moon is the third full moon in a season that has four full moons. Over the next 20 years there will be about 15 blue moons, with an almost equal number of both types of blue moons occurring. No blue moon of any kind will occur in the years 2011, 2014, and 2017.
In some myths and regional stories, GrandMother Moon turns blue on rare occasions. The reasons for change vary from place to place and culture to culture. In days of old, bad weather was often an omen that Mother Earth was angry or being harmed or attacked in some way. If this happened during a blue moon, the legends explained that GrandMother Moon would become sad (blue) because her daughter had been wounded.
In other stories, GrandMother Moon becomes concerned that Mother Earth was sad over some tragic event that befalls her consort. Or still other variations, GrandMother turned blue because she herself had become angered by some event. These stories are unique because the hue of the Blue Moon is even more rare than the blue moon event itself.
The colour blue and its importance was derived from the skin colour of Lord Krishna, who is revered as the divine flute player. Krishna is the special energy of the second Moon was considered 'blue' or divine by Indian religious scholars and priests. Thus they celebrate two Full Moons in the one sign and had large religious and sacrificial ceremonies to acknowledge the importance of the second.
Lunar energies of the blue moon are much stronger than usual, and many spells and workings are stronger during those times.