The lunar calendar is used because of the lunar phase and the cycles from those phases are what they base the calendar on. It is known more for being the Islamic calendar or either the Hijri calendar. This calendar always has 12 months in it. This calendar is mainly used for religious reasons and can be used for other reasons such as for commercial which is what Saudi Arabia often uses it for. It is a face that these calendars is the lunisolar calendar, meaning that the lunar cycle is kept by months and then the months that would be intercalary would go in synchronization with solar years. The days in the lunar calendar is usually 354.37 days.

The Hindu, Hebrew, and the Chinese also use the lunar calendar with antiquity. The start of the lunar month varies depending on what calendar is being used. The full moon is the start of a new month for the Hindu but the Hebrew uses the lunar crescent for the new month. The Chinese calendar uses the new moon for the first day of the month.

Another thing that actually varies is the length with the lunar month. Most months have at least 29 to 30 days in them depending on the moon and other things. A full and hollow month is the factor used to determine the length of the month. Whatever calendar is being used things change each year and that is why you always have to buy a new calendar when the year ends.