A blood moon, or lunar tetrad, does not occur on a regular schedule. However, the blood moons of 2014 and 2015 are not isolated events. Between the first century and 2014, there were 62 lunar tetrads.
Lunar tetrads occur when a series of four lunar eclipses happen six months from each other. Lunar tetrads were not referred to as blood moons until the 21st century when Christian author John Hagee popularized the term in his 2013 book "Four Blood Moons".
During a lunar eclipse, the Earth is directly between the sun and the moon. The shadow cast by the earth onto the moon during an eclipse gives the moon a copper-colored appearance.