Lunar calendars are used by people of various cultures and chiefly the days of the calendar are determined by the moon phase. The normal procedure used to determine a lunar month is to consider a month as a lunation in which the days of the month are indicated as a part of phase of moon. The lunisolar calendars also tracks the seasons. In our normal calendars we have 365 days and 366 in the case of a leap year. The lunar calendars on the other hand consist of 354.37 days, also referred to as the lunar year, the twelve months being denoted as synodic months.
A vide number of cultures do depend upon the lunar year. To name a few is the Chinese Calendar, the Hindu calendar and the Hebrew Calendar. Most of the calendar systems are used in antiquity, the basic reason being that the number of days in a lunar year is not an exact multiple of 12 which equals the number of lunation in a lunar year. Therefore a correction of the calendars is very much required, and should that not be included, the lunar calendar year shall keep on drifting every year with respect to the different seasons.
Islamic Calendar is a very popular example of the lunar calendar and is very popular all round the globe. The number of lunation in the Islamic calendar is always equal to twelve and is therefore used for religious purposes.
The rise of the new moon decides the first day of each month for the Chinese calendar and is more accurately defined with respect to a time zone. Quite many other lunar calendars do determine the first day of the month by sighting the lunar crescent.
Generally, the length of synodic months equals to 29.530589 days with the length of each alternate month being 29 and 30 days, also popularly referred to as hollow and full months. Fixed arithmetical rules have been devised to reduce error from calculations made by astronomical observations. The distribution of hollow and full months is determined by use of continued fractions. In 1700 AD Isaac Newton derived the Easter computation so as to determine a 49 month cycle of the lunar cycle.