Calendar

== History ==
{{Main|History of calendars}}
{{Further|Week|Calendar epoch|Month|Lunisolar calendar|Computus|Calendar reform}}
[[File:Equinozio da Pizzo Vento,tramonto fondachelli fantina, sicilia.JPG|thumb|[[Equinox]] seen from the astronomic calendar of Pizzo Vento at [[Fondachelli-Fantina|Fondachelli Fantina]], [[Sicily]]]]

The course of the Sun and the Moon are the most salient regularly recurring natural events useful for [[timekeeping]], and in pre-modern societies around the world [[lunation]] and the [[year]] were most commonly used as time units. Nevertheless, the Roman calendar contained remnants of a very ancient pre-Etruscan 10-month solar year.<ref>"Religion in the Etruscan Period" in [http://global.britannica.com/topic/Roman-religion Roman Religion] {{Webarchive|url=https://web.archive.org/web/20151015213053/http://global.britannica.com/topic/Roman-religion |date=15 October 2015 }} in the ''Encyclopædia Britannica''</ref> 

The first recorded physical calendars, dependent on the development of [[Bronze Age writing|writing]] in the [[Ancient Near East]], are the [[Bronze Age]] [[Egyptian calendar|Egyptian]] and [[Sumerian calendar|Sumerian]] calendars.<ref>{{Cite journal|last=Shirley|first=Lawrence|date=11 February 2009|title=The Mayan and Other Ancient Calendars|url=http://dx.doi.org/10.4169/loci003264|location=Washington, DC|doi=10.4169/loci003264|journal=Convergence|access-date=16 March 2021|archive-date=9 April 2023|archive-url=https://web.archive.org/web/20230409163211/https://www.maa.org/press/periodicals/convergence/the-mayan-and-other-ancient-calendars|url-status=live}}</ref>

During the [[Vedic period]] India developed a sophisticated timekeeping methodology and calendars for [[Vedas|Vedic]] rituals.<ref>{{Cite book|last=Plofker|first=Kim|title=Mathematics in India|publisher=Princeton University Press|year=2009|isbn=978-0-691-12067-6|pages=10, 35-36, 67}}</ref> According to Yukio Ohashi, the Vedanga calendar in ancient India was based on astronomical studies during the Vedic Period and was not derived from other cultures.<ref>{{Cite book|last=Andersen|first=Johannes|url=https://books.google.com/books?id=gQYscrT0fgQC|title=Highlights of Astronomy, Volume 11B: As Presented at the XXIIIrd General Assembly of the IAU, 1997|date=31 January 1999|publisher=Springer Science & Business Media|isbn=978-0-7923-5556-4|pages=719|language=en|access-date=12 March 2021|archive-date=11 January 2023|archive-url=https://web.archive.org/web/20230111163620/https://books.google.com/books?id=gQYscrT0fgQC|url-status=live}}</ref>

A large number of calendar systems in the Ancient Near East were based on the [[Babylonian calendar]] dating from the [[Iron Age]],<ref>{{Cite book |url=https://books.google.com/books?id=cg0OAQAAMAAJ&newbks=0&printsec=frontcover&dq=A+large+number+of+calendar+systems+in+the+Ancient+Near+East+were+based+on+the+Babylonian+calendar+dating+from+the+Iron+Age&q=A+large+number+of+calendar+systems+in+the+Ancient+Near+East+were+based+on+the+Babylonian+calendar+dating+from+the+Iron+Age&hl=vi |title=Civilizations of the Ancient Near East |date=1995 |publisher=Scribner |isbn=978-0-684-19279-6 |language=en}}</ref> among them the calendar system of the Persian Empire, which in turn gave rise to the [[Zoroastrian calendar]] and the [[Hebrew calendar]].<ref>{{Cite web |date=2021-09-12 |title=Egyptians celebrate new Egyptian year on September 11 |url=https://www.egypttoday.com/Article/4/107766/Egyptians-celebrate-new-Egyptian-year-on-September-11 |access-date=2024-03-12 |website=EgyptToday}}</ref><ref>{{Cite book |last=Sayeed |first=Ahmed |url=https://books.google.com/books?id=GVeoDwAAQBAJ&newbks=0&printsec=frontcover&pg=PT407&dq=calendar+system+of+the+Persian+Empire,+which+in+turn+gave+rise+to+the+Zoroastrian+calendar+and+the+Hebrew+calendar.&hl=vi |title=You Must Win: The winner can create History |date=2019-08-10 |publisher=Prowess Publishing |isbn=978-1-5457-4730-8 |language=en}}</ref> 

A great number of [[Hellenic calendars]] were developed in [[Classical Greece]], and during the [[Hellenistic period]] they gave rise to the ancient [[Roman calendar]] and to various [[Hindu calendars]].<ref>{{Cite web|title=Calendar - The Early Roman Calendar|url=https://www.britannica.com/science/calendar|access-date=19 January 2021|date=24 December 2020|website=Encyclopedia Britannica|language=en|archive-date=4 January 2021|archive-url=https://web.archive.org/web/20210104181246/https://www.britannica.com/science/calendar|url-status=live}}</ref> 

Calendars in antiquity were [[lunisolar]], depending on the introduction of [[intercalary month]]s to align the solar and the lunar years. This was mostly based on observation, but there may have been early attempts to model the pattern of intercalation algorithmically, as evidenced in the fragmentary 2nd-century [[Coligny calendar]].

The [[Roman calendar]] was reformed by [[Julius Caesar]] in 46&nbsp;BC.<ref>{{cite web|title=The History of the Calendar|url=https://www.calendar.com/history-of-the-calendar/ |archive-url=https://web.archive.org/web/20201202131025/https://www.calendar.com/history-of-the-calendar/ |archive-date=2020-12-02 |url-status=dead}}</ref> His "Julian" calendar was no longer dependent on the observation of the new moon, but followed an algorithm of introducing a leap day every four years. This created a dissociation of the calendar [[month]] from [[lunation]]. The [[Gregorian calendar]], introduced in 1582, corrected most of the remaining difference between the Julian calendar and the solar year.

The [[Islamic calendar]] is based on the prohibition of intercalation (''[[nasi']]'') by [[Muhammad]], in Islamic tradition dated to a sermon given on 9 [[Dhu al-Hijjah]] AH&nbsp;10 (Julian date: 6 March 632). This resulted in an observation-based lunar calendar that shifts relative to the seasons of the solar year.

There have been several modern proposals for reform of the modern calendar, such as the [[World Calendar]], the [[International Fixed Calendar]], the [[Holocene calendar]], and the [[Hanke–Henry Permanent Calendar]]. Such ideas are mooted from time to time, but have failed to gain traction because of the loss of continuity and the massive upheaval that implementing them would involve, as well as their effect on cycles of religious activity.