A calendar is a system of organizing days for social, religious, commercial, or administrative purposes. This is done by giving names to periods of time, typically days, weeks, months, and years. The name given to each day is known as a date. Periods in a calendar (such as years and months) are usually, though not necessarily, synchronized with the cycle of the sun or the moon. Many civilizations and societies have devised a calendar, usually derived from other calendars on which they model their systems, suited to their particular needs.
A calendar is also a physical device (often paper). This is the most common usage of the word. Other similar types of calendars can include computerized systems, which can be set to remind the user of upcoming events and appointments.
A calendar can also mean a list of planned events, such as a court calendar.
The English word calendar is derived from the Latin word kalendae, which was the Latin name of the first day of every month.
 Calendar systems
A full calendar system has a different calendar date for every day. Thus the week cycle is by itself not a full calendar system; neither is a system to name the days within a year without a system for identifying the years.
The simplest calendar system just counts time periods from a reference date. This applies for the Julian day. Virtually the only possible variation is using a different reference date, in particular one less distant in the past to make the numbers smaller. Computations in these systems are just a matter of addition and subtraction.
Other calendars have one (or multiple) larger units of time.
Calendars that contain one level of cycles:
- week and weekday ' this system (without year, the week number keeps on increasing) is not very common
- year and ordinal date within the year, e.g. the ISO 8601 ordinal date system
Calendars with two levels of cycles:
Cycles can be synchronized with periodic phenomena:
- A lunar calendar is synchronized to the motion of the Moon (lunar phases); an example is the Islamic calendar.
- A solar calendar is based on perceived seasonal changes synchronized to the apparent motion of the Sun; an example is the Persian calendar.
- A "luni-solar calendar" is based on a combination of both solar and lunar reckonings; an example is the traditional calendar of China and the Hindu Calendar in India.
- There are some calendars that appear to be synchronized to the motion of Venus, such as some of the ancient Egyptian calendars; synchronization to Venus appears to occur primarily in civilizations near the Equator.
- The week cycle is an example of one that is not synchronized to any external phenomenon (although it may have been derived from lunar phases, beginning anew every month).
Very commonly a calendar includes more than one type of cycle, or has both cyclic and acyclic elements. A lunisolar calendar is synchronized both to the motion of the moon and to the apparent motion of the sun; an example is the Hebrew calendar.
Many calendars incorporate simpler calendars as elements. For example, the rules of the Hebrew calendar depend on the seven-day week cycle (a very simple calendar), so the week is one of the cycles of the Hebrew calendar. It is also common to operate two calendars simultaneously, usually providing unrelated cycles, and the result may also be considered a more complex calendar. For example, the Gregorian calendar has no inherent dependence on the seven-day week, but in Western society the two are used together, and calendar tools indicate both the Gregorian date and the day of week.
The week cycle is shared by various calendar systems (although the significance of special days such as Friday, Saturday, and Sunday varies). Systems of leap days usually do not affect the week cycle. The week cycle was not even interrupted when 10, 11, 12, or 13 dates were skipped when the Julian calendar was replaced by the Gregorian calendar by various countries.
 Solar calendars
Main article: Solar calendar
 Days used by solar calendars
Solar calendars assign a date to each solar day. A day may consist of the period between sunrise and sunset, with a following period of night, or it may be a period between successive events such as two sunsets. The length of the interval between two such successive events may be allowed to vary slightly during the year, or it may be averaged into a mean solar day. Other types of calendar may also use a solar day.
 Calendar reform
There have been a number of proposals for reform of the calendar, such as the World Calendar, International Fixed Calendar and Holocene calendar. The United Nations considered adopting such a reformed calendar for a while in the 1950s, but these proposals have lost most of their popularity.
 Lunar calendars
Main article: Lunar calendar
Not all calendars use the solar year as a unit. A lunar calendar is one in which days are numbered within each lunar phase cycle. Because the length of the lunar month is not an even fraction of the length of the tropical year, a purely lunar calendar quickly drifts against the seasons, which don't vary much near the equator. It does, however, stay constant with respect to other phenomena, notably tides. An example is the Islamic calendar. Alexander Marshack, in a controversial reading, believed that marks on a bone baton (c. 25,000 BC) represented a lunar calendar. Other marked bones may also represent lunar calendars. Similarly, Michael Rappenglueck believes that marks on a 15,000-year old cave painting represent a lunar calendar.
 Lunisolar calendars
A lunisolar calendar is a lunar calendar that compensates by adding an extra month as needed to realign the months with the seasons. An example is the Hebrew calendar which uses a 19-year cycle.
 Calendar subdivisions
Nearly all calendar systems group consecutive days into "months" and also into "years". In a solar calendar a year approximates Earth's tropical year (that is, the time it takes for a complete cycle of seasons), traditionally used to facilitate the planning of agricultural activities. In a lunar calendar, the month approximates the cycle of the moon phase. Consecutive days may be grouped into other periods such as the week.
Because the number of days in the tropical year is not a whole number, a solar calendar must have a different number of days in different years. This may be handled, for example, by adding an extra day (29 February) in leap years. The same applies to months in a lunar calendar and also the number of months in a year in a lunisolar calendar. This is generally known as intercalation. Even if a calendar is solar, but not lunar, the year cannot be divided entirely into months that never vary in length.
Cultures may define other units of time, such as the week, for the purpose of scheduling regular activities that do not easily coincide with months or years. Many cultures use different baselines for their calendars' starting years. For example, the year in Japan is based on the reign of the current emperor: 2006 was Year 18 of the Emperor Akihito.
See Decade, Century, Millennium
 Other calendar types
 Arithmetic and astronomical calendars
An astronomical calendar is based on ongoing observation; examples are the religious Islamic calendar and the old religious Jewish calendar in the time of the Second Temple. Such a calendar is also referred to as an observation-based calendar. The advantage of such a calendar is that it is perfectly and perpetually accurate. The disadvantage is that working out when a particular date would occur is difficult.
An arithmetic calendar is one that is based on a strict set of rules; an example is the current Jewish calendar. Such a calendar is also referred to as a rule-based calendar. The advantage of such a calendar is the ease of calculating when a particular date occurs. The disadvantage is imperfect accuracy. Furthermore, even if the calendar is very accurate, its accuracy diminishes slowly over time, owing to changes in Earth's rotation. This limits the lifetime of an accurate arithmetic calendar to a few thousand years. After then, the rules would need to be modified from observations made since the invention of the calendar.
 Complete and incomplete calendars
Calendars may be either complete or incomplete. Complete calendars provide a way of naming each consecutive day, while incomplete calendars do not. The early Roman calendar, which had no way of designating the days of the winter months other than to lump them together as "winter", is an example of an incomplete calendar, while the Gregorian calendar is an example of a complete calendar.
The primary practical use of a calendar is to identify days: to be informed about and/or to agree on a future event and to record an event that has happened. Days may be significant for civil, religious or social reasons. For example, a calendar provides a way to determine which days are religious or civil holidays, which days mark the beginning and end of business accounting periods, and which days have legal significance, such as the day taxes are due or a contract expires. Also a calendar may, by identifying a day, provide other useful information about the day such as its season.
Calendars are also used to help people manage their personal schedules, time and activities, particularly when individuals have numerous work, school, and family commitments. People frequently use multiple systems, and may keep both a business and family calendar to help prevent them from overcommitting their time.
Calendars are also used as part of a complete timekeeping system: date and time of day together specify a moment in time. In the modern world, written calendars are no longer an essential part of such systems, as the advent of accurate clocks has made it possible to record time independently of astronomical events.
 Currently used calendars
Calendars in widespread use today include the Gregorian calendar, which is the de facto international standard, and is used almost everywhere in the world for civil purposes, including in the People's Republic of China and India (along with the Indian national calendar). Due to the Gregorian calendar's obvious connotations of Western Christianity, non-Christians and even some Christians sometimes justify its use by replacing the traditional era notations "AD" and "BC" ("Anno Domini" and "Before Christ") with "CE" and "BCE" ("Common Era" and "Before Common Era"). The Hindu calendars are some of the most ancient calendars of the world. Eastern Christians of eastern Europe and western Asia used for a long time the Julian Calendar, that of the old Orthodox church, in countries like Russia. For over 1500 years, Westerners used the Julian Calendar also.
While the Gregorian calendar is widely used in Israel's business and day-to-day affairs, the Hebrew calendar, used by Jews worldwide for religious and cultural affairs, also influences civil matters in Israel (such as national holidays) and can be used there for business dealings (such as for the dating of checks).
The Iranian (Persian) calendar is used in Iran and Afghanistan. The Islamic calendar is used by most non-Iranian Muslims worldwide. The Chinese, Hebrew, Hindu, and Julian calendars are widely used for religious and/or social purposes. The Ethiopian calendar or Ethiopic calendar is the principal calendar used in Ethiopia and Eritrea. In Thailand, where the Thai solar calendar is used, the months and days have adopted the western standard, although the years are still based on the traditional Buddhist calendar. Bah�¡'ís worldwide use the Bah�¡'í calendar.
Even where there is a commonly used calendar such as the Gregorian calendar, alternate calendars may also be used, such as a fiscal calendar or the astronomical year numbering system.
 Fiscal calendars
A fiscal calendar (such as a 4/4/5 calendar) fixes each month at a specific number of weeks to facilitate comparisons from month to month and year to year. January always has exactly 4 weeks (Sunday through Saturday), February has 4 weeks, March has 5 weeks, etc. Note that this calendar will normally need to add a 53rd week to every 5th or 6th year, which might be added to December or might not be, depending on how the organization uses those dates. There exists an international standard way to do this (the ISO week). The ISO week starts on a Monday, and ends on a Sunday. Week 1 is always the week that contains 4 January in the Gregorian calendar.
Fiscal calendars are also used by businesses. This is where the fiscal year is just any set of 12 months. This set of 12 months can start and end at any point on the Gregorian calendar. This is the most common usage of fiscal calendars.
 Gregorian calendar with Easter Sunday
Calculating the calendar of a previous year (for the Gregorian calendar taking account of the week) is a relatively easy matter when Easter Sunday is not included on the calendar. However, calculating for Easter Sunday is difficult because the calculation requires the knowledge of the full moon cycle. Easter Sunday is on the first Sunday after the first full moon after the Vernal Equinox according to the computus. So, this makes an additional calculation necessary on top of the normal calculation for January 1 and the calculation of whether or not the year is a leap year.
There are only 14 different calendars when Easter Sunday is not involved. Each calendar is determined by the day of the week January 1 falls on and whether or not the year is a leap year. However, when Easter Sunday is included, there are 70 different calendars (two for each date of Easter).
 Physical calendars
A calendar is also a physical device (often paper) (for example, a desktop calendar or a wall calendar). In a paper calendar one or two sheets can show a single day, a week, a month, or a year. If a sheet is for a single day, it easily shows the date and the weekday. If a sheet is for multiple days it shows a conversion table to convert from weekday to date and back. With a special pointing device, or by crossing out past days, it may indicate the current date and weekday. This is the most common usage of the word.
The sale of physical calendars has been restricted in some countries, and given as a monopoly to universities and national academies. Examples include the Prussian Academy of Sciences and the University of Helsinki, which had a monopoly on the sale of calendars in Finland until the 1990s.
Main article: Docket (court)
For lawyers and judges, the calendar is the docket used by the court to schedule the order of hearings or trials. A paralegal or court officer may keep track of the cases by using docketing software.
 Calendars in computing
There are different layouts for calendars.
A calendar which has a different month on each page. This page shows August
 See also
 List of calendars
- Birashk, Ahmad (1993), A comparative Calendar of the Iranian, Muslim Lunar, and Christian Eras for Three Thousand Years, Mazda Publishers, ISBN 0939214954
- Dershowitz, Nachum; Reingold, Edward M (1997), Calendrical Calculations, Cambridge University Press, ISBN 0521564743, http://emr.cs.iit.edu/home/reingold/calendar-book/second-edition/ with Online Calculator
- Zerubavel, Eviatar (1985), The Seven Day Circle: The History and Meaning of the Week, University of Chicago Press, ISBN 0226981657
- Doggett, LE (1992), "Calendars", in Seidelmann, P. Kenneth, Explanatory Supplement to the Astronomical Almanac, University Science Books, ISBN 0935702687, http://astro.nmsu.edu/~lhuber/leaphist.html
- Árni Björnsson (1995) , High Days and Holidays in Iceland, Reykjavík: M�¡l og menning, ISBN 9979308028, OCLC 186511596
- Richards, EG (1998), Mapping Time, the calendar and its history, Oxford University Press, ISBN 0198504136
- Rose, Lynn E (1999), Sun, Moon, and Sothis, Kronos Press, ISBN 0917994159
- Spier, Arthur (1986), The Comprehensive Hebrew Calendar, Feldheim Publishers, ISBN 0873063988
- Dieter Schuh (1973) (in German), Untersuchungen zur Geschichte der Tibetischen Kalenderrechnung, Wiesbaden: Franz Steiner Verlag, OCLC 1150484
 Further reading
- Fraser, Julius Thomas (1987), Time, the Familiar Stranger (illustrated ed.), Amherst: Univ of Massachusetts Press, ISBN 0870235761, OCLC 15790499
- Whitrow, Gerald James (2003), What is Time?, Oxford: Oxford University Press, ISBN 0198607814, OCLC 265440481
 External links