A calendar is a system of organizing time in terms of days, weeks, months and years for the purpose of measuring and recording time over extended periods. The basis of most calendars, including the six principal ones currently in use, is astronomical. A day is based on the rotation of the Earth on its axis; a year is based on the revolution of the Earth around the sun while a month is based on the revolution of the Moon around the Earth.http://www.usno.navy.mil/USNO/astronomical-applications/astronomical-information-center/calendars A week is the only artificial division of time without any astronomical basis.http://www.bbc.co.uk/dna/h2g2/A698989

The Julian and Gregorian calendar are basically solar calendars whereas the Hebrew, Indian and Chinese calendar are lunisolar calendars. The Islamic calendar is purely lunar in nature.http://www.usno.navy.mil/USNO/astronomical-applications/astronomical-information-center/calendars Each month of a lunar calendar follows a full cycle of the Moon’s phases. Not relating to the motion of the sun, a lunar calendar requires adding a whole month’s time every few years to bring the seasons back in phase. In agricultural societies, timing the seasons accurately is a matter of life and death. Such modifications of the lunar calendar gave rise to the lunisolar calendar.http://charon.nmsu.edu/~lhuber/leaphist.html A solar calendar, including the widely used Gregorian calendar, requires its own adjustments because a year does not comprise an integral number of days or lunar months.http://www.usno.navy.mil/USNO/astronomical-applications/astronomical-information-center/calendars

The revolution of the Earth around the sun takes 365.242199 days. To minimize error in the daily use of the Gregorian calendar, years are divided into two classes: the common years with 365 days and the leap years with 366 days. An intercalary day of February 29 is added to a leap year which is exactly divisible by four except for years that are exactly divisible by 100. These centurial years are leap years only if they are exactly divisible by 400. In this way, the Gregorian calendar accumulates an error of one day in about 2500 years.http://charon.nmsu.edu/~lhuber/leaphist.html