Computus week before the Gregorian Easter in 1724, 1744, 1778, 1798, etc. The Easter cycle groups days into lunar months, which are either 29 or 30 days long. There is an exception. The month ending in March normally has thirty days, but if 29 February of a leap year falls within it, it contains 31. As these groups are based on the lunar cycle, over the long term the average month in the lunar calendar is a very good approximation of the synodic month, which is days long. There are 12 synodic months in a lunar year, totaling either 354 or
Lunar month has been observed among cultures in the Middle East, India, and China in the following way: they divided the sky into 27 or 28 lunar mansions, one for each day of the month, identified by the prominent star(s) in them. The synodic month (, "synodikós", meaning "pertaining to a synod, i.e., a meeting"; in this case, of the Sun and the Moon) is the average period of the Moon's orbit with respect to the line joining the Sun and Earth. This is the period of the lunar phases, because the Moon's appearance depends on the position of the Moon with