Blue Moon, What Is It? ... Moon | ... By popular acclaim, the second Full Moon of a calendar month is called a Blue Moon. ... If the last Full Moon of this calendar year comes after the December solstice but before New Year's Day, then it's inevitable -- the following year will have an "Old Style" Blue Moon. ... There are always thirteen Full Moons in a calendar year that hosts a "New Style" Blue Moon, and almost always twelve Full Moons during a calendar year that features an "Old Style" Blue Moon. ...