Calendar

==Usage==
[[File:World calendars map.svg|thumb|center|800px|Current usage of civil calendars in the world. Some countries use an additional calendar not listed here that determine holidays and other traditions, for example the [[Chinese calendar|Chinese]] or [[Islamic calendar|Islamic]] calendars.]]
The primary practical use of a calendar is to identify days: to be informed about or to agree on a future event and to record an event that has happened. Days may be significant for agricultural, civil, religious, or social reasons. For example, a calendar provides a way to determine when to start planting or harvesting, which days are [[religious holiday|religious]] or [[civil holiday]]s, 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 as part of a complete [[timekeeping]] system: date and time of day together specify a moment in [[time]].  In the modern world, timekeepers can show time, date, and weekday. Some may also show the lunar phase.

===Gregorian===
The [[Gregorian calendar]] is the ''[[de facto]]'' international standard and is used almost everywhere in the world for civil purposes. The widely-used solar aspect is a cycle of leap days in a 400-year cycle designed to keep the duration of the year aligned with the [[solar year]].{{sfn | Blackburn | Holford-Strevens | 2003 | pp = 682&ndash;683}} There is a lunar aspect which approximates the position of the moon during the year, and is used in the [[Computus | calculation of the date of Easter]].{{sfn | Blackburn | Holford-Strevens | 2003 | pp = 817&ndash;820}} Each Gregorian year has either 365 or 366 days (the leap day being inserted as 29 February), amounting to an average Gregorian year of 365.2425 days (compared to a solar year of 365.2422 days).{{sfn | Dershowitz | Reingold | 2008 | pp = 47, 187}}

The calendar was introduced in 1582 as a refinement to the [[Julian calendar]], which had been in use throughout the European Middle Ages, amounting to a 0.002% correction in the length of the year.{{sfn | Blackburn | Holford-Strevens | 2003 | pp = 682&ndash;683}} During the Early Modern period, [[Adoption of the Gregorian calendar|its adoption was mostly limited]] to [[Roman Catholic]] nations, but by the 19th century it had become widely adopted for the sake of convenience in international trade. The last European country to adopt it was Greece, in 1923.{{sfn | Blackburn | Holford-Strevens | 2003 | pp = 682&ndash;689}}

The [[calendar epoch]] used by the Gregorian calendar is inherited from the medieval convention established by [[Dionysius Exiguus]] and associated with the Julian calendar. The year number is variously given as AD (for ''[[Anno Domini]]'') or CE (for ''[[Common Era]]'' or ''Christian Era'').{{sfn | Blackburn | Holford-Strevens | 2003 | loc= Chapter: "Christian Chronology"}}

===Religious===
[[File:Hindu calendar 1871-72.jpg|thumb|A Hindu [[almanac]] (''pancanga'') for the year 1871/2 from [[Rajasthan]] (Library of Congress, Asian Division)]]
The most important use of pre-modern calendars is keeping track of the [[liturgical year]] and the observation of religious feast days.

While the Gregorian calendar is itself historically motivated to the calculation of the [[Easter date]], it is now in worldwide secular use as the ''de facto'' standard. Alongside the use of the Gregorian calendar for secular matters, there remain several calendars in use for religious purposes.

Western Christian liturgical calendars are based on the cycle of the Roman Rite of the Catholic Church and generally include the liturgical seasons of [[Advent]], [[Christmas]], Ordinary Time (Time after [[Epiphany (holiday)|Epiphany]]), [[Lent]], [[Easter]], and Ordinary Time (Time after [[Pentecost]]). Some Christian calendars do not include Ordinary Time and every day falls into a denominated season.

[[Eastern Christians]], including the [[Eastern Orthodox Church|Orthodox Church]], use the [[Julian calendar]].

The [[Islamic calendar]] or Hijri calendar is a [[lunar calendar]] consisting of 12 lunar months in a year of 354 or 355 days. It is used to date events in most of the Muslim countries (concurrently with the Gregorian calendar) and used by Muslims everywhere to determine the proper day on which to celebrate Islamic holy days and festivals. Its epoch is the [[Anno Hegirae|Hijra]] (corresponding to AD&nbsp;622). With an annual drift of 11 or 12 days, the seasonal relation is repeated approximately every 33 Islamic years.

Various [[Hindu calendar]]s remain in use in the Indian subcontinent, including the [[Nepali calendar (disambiguation)|Nepali calendars]], [[Bengali calendar]], [[Malayalam calendar]], [[Tamil calendar]], [[Vikrama Samvat]] used in Northern India, and [[Shalivahana era|Shalivahana]] calendar in the [[Deccan Plateau|Deccan]] states.

The [[Buddhist calendar]] and the traditional lunisolar calendars of [[Cambodia]], [[Laos]], [[Myanmar]], [[Sri Lanka]] and [[Thailand]] are also based on an older version of the Hindu calendar.

Most of the Hindu calendars are inherited from a system first enunciated in [[Vedanga Jyotisha]] of Lagadha, standardized in the ''[[Surya Siddhanta|Sūrya Siddhānta]]'' and subsequently reformed by astronomers such as [[Aryabhata|Āryabhaṭa]] (AD&nbsp;499), [[Varahamihira|Varāhamihira]] (6th century) and [[Bhāskara II]] (12th century).

The [[Hebrew calendar]] is used by [[Jews]] worldwide for religious and cultural affairs, also influences civil matters in Israel (such as [[public holiday|national holidays]]) and can be used business dealings (such as for the dating of [[cheque]]s).<ref>{{Cite web |title=About the Hebrew Calendar {{!}} Yale University Library |url=https://web.library.yale.edu/cataloging/hebraica/about-hebrew-calendar |access-date=2024-03-18 |website=web.library.yale.edu}}</ref>

Followers of the [[Baháʼí Faith]] use the [[Baháʼí calendar]]. The Baháʼí Calendar, also known as the Badi Calendar was first established by the Bab in the Kitab-i-Asma. The Baháʼí Calendar is also purely a solar calendar and comprises 19 months each having nineteen days.

===National===
The [[Chinese calendar|Chinese]], [[Hebrew calendar|Hebrew]], [[Hindu calendar|Hindu]], and [[Julian calendar|Julian]] calendars are widely used for religious and social purposes.

The [[Solar Hijri calendar|Iranian (Persian) calendar]] is used in [[Iran]] and some parts of [[Afghanistan]]. The [[Assyrian calendar]] is in use by the members of the Assyrian community in the Middle East (mainly Iraq, Syria, Turkey, and Iran) and the diaspora. The first year of the calendar is exactly 4750 years prior to the start of the Gregorian calendar. The [[Ethiopian calendar]] or Ethiopic calendar is the principal calendar used in [[Ethiopia]] and [[Eritrea]], with the [[Oromo calendar]] also in use in some areas. In neighboring [[Somalia]], the [[Somali calendar]] co-exists alongside the Gregorian and Islamic calendars. 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]].

===Fiscal===
{{Main|Fiscal calendar}}
[[File:Pieter Brueghel the Younger (or workshop) The Payment of the Tithes Bonhams.jpg|thumb|''The Payment of the Tithes'' (The tax-collector), also known as ''Village Lawyer'', by [[Pieter Brueghel the Younger]] ]]

A fiscal calendar generally means the accounting year of a government or a business. It is used for budgeting, keeping accounts, and taxation. It is a set of 12 months that may start at any date in a year. The US government's fiscal year starts on 1 October and ends on 30 September. The government of India's fiscal year starts on 1 April and ends on 31 March. Small traditional businesses in India start the fiscal year on [[Diwali]] festival and end the day before the next year's Diwali festival.

In accounting (and particularly accounting software), a fiscal calendar (such as a [[4–4–5 calendar|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 8601|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.