A Twelve-Month Calendar based primarily on the motions of the Earth and the Moon with respect to each other (a Lunar Calendar); for example, the month is defined by the revolution of the Moon around the Earth, which takes approx. 29 and a half days. Over twelve months, this would produce a year of approx. 354 days. In order to satisfy the Biblical requirement that Nisan, the First Month, be in the Springtime, a thirteenth month (Second Adar) is inserted in certain years.