 In the Hebrew Calendar this is noted with a periodic extra month of Adar in the early spring.In the modern system of "traditional" American full moon names tied to the solstice and equinox points, a supernumerary full moon in such a period is called a blue moon.The term "blue moon" used in this sense may date to as early as the 16th century, but it became well-known in the United States due to the "Farmers' Almanac" (published since 1818).