1. Calendar
2012

2. Origin and history
C.1200, "system of division of the year;" mid-14c. as "table showing divisions of the year;" from O.Fr. calendier "list, register," from L. calendarium "account book," from calendae/kalendae "calends" the first day of the Roman month -- when debts fell due and accounts were reckoned -- from calare "to announce solemnly, call out," as the priests did in proclaiming the new moon that marked the calends, from PIE base kele- "to call, shout" (see claim). Taken by the early Church for its register list of saints and their feast days. The -ar spelling in English is 17c. to differentiate it from the now obscure calender "cloth-presser" (see calender).
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3. Types of calendars
The motion (or apparent motion) of the Sun and the Moon provide temporal cycles which have strongly influenced the design of most calendars, which usually attempt to accord either with the solar cycle (the cycle of the seasons) or with the lunar cycle (the cycle of the phases of the Moon) or with both. There are over forty calendars currently in use, and many others that have been used or have been invented. They thus come in many forms. They may be divided into six kinds:
1. Purely lunar calendars
2. Purely solar calendars
3. Lunisolar calendars
4. Solilunar calendars
5. Solar-count lunar calendars
6. Other calendars

4. Most used calendars Chinese calendar Gregorian calendar
the former calendar of China, in which the year consisted of 12 lunar months with an intercalary month added seven times every 19 years to reconcile the lunar year of 354 days with the solar year of 365 days, time being reckoned in 60-year cycles with the first cycle dating from 2637 b.c.
Gregorian calendar 
the reformed Julian calendar now in use, according to which the ordinary year consists of 365 days, and a leap year of 366 days occurs in every year whose number is exactly divisible by 4 except centenary years whose numbers are not exactly divisible by 400, as 1700, 1800, and 1900.

7. Hindu calendar Jewish calendar
a lunisolar calendar that governs all Hindu and most Indian festivals, known from about 1000 b.c. and subsequently modified during the 4th and 6th centuries a.d.
Jewish calendar 
the lunisolar calendar used by Jews, as for determining religious holidays, that is reckoned from 3761 b.c. and was established by Hillel II in the 4th century a.d., the calendar year consisting of 353 days (defective year), 354 days (regular year), or 355 days (perfect year or abundant year) and containing 12 months: Tishri, Heshvan, Kislev, Tevet, Shevat, Adar, Nisan, Iyar, Sivan, Tammuz, Av, and Elul, with the 29-day intercalary month of Adar Sheni added after Adar seven times in every 19-year cycle in order to adjust the calendar to the solar cycle. The Jewish ecclesiastical year begins with Nisan and the civil year with Tishri.

10. Julian calendar Muslim calendar
the calendar established by Julius Caesar in 46 b.c., fixing the length of the year at 365 days and at 366 days every fourth year. There are 12 months of 30 or 31 days, except for February (which has 28 days with the exception of every fourth year, or leap year, when it has 29 days).
Muslim calendar 
the lunar calendar used by Muslims and reckoned from a.d. 622: the calendar year consists of 354 days and contains 12 months: Moharram, Safar, Rabi I, Rabi II, Jumada I, Jumada II, Rajab, Shaban, Ramadan, Shawwal, Dhu ʾl-Qaʿda, and Dhu ʾl-hijjah. In leap years the month Dhu ʾl-hijjah contains one extra day.

13. Ecclesiastical calendar
1. a calendar based on the lunisolar cycle, used by many Christian churches in determining the dates for the movable feasts. Also called church calendar. a calendar of the Christian year, indicating the days and seasons for fasts and festivals.