Ancient cultures had different "week" lengths, including ten in Egypt and an eight-day week for [[Etruscan civilization|Etruscans]]. The Etruscan week was adopted by the [[Ancient Rome|ancient Romans]], but they later moved to a seven-day week, which had spread across Western Asia and the Eastern Mediterranean due to the influence of the Christian seven-day week, which is rooted in the Jewish seven-day week. In 321 CE, Emperor [[Constantine the Great]] officially decreed a seven-day week in the Roman Empire, including making Sunday a public holiday.<ref>{{Cite web |last=Ring |first=Rosanna |date=22 January 2021 |title=A history of time – the story behind our days, weeks, and months |url=https://www.stneotsmuseum.org.uk/articles/a-history-of-time-the-story-behind-our-days-weeks-and-months/ |access-date=6 January 2023 |website=St Neots Museum |language=en-GB}}</ref><ref>[https://www.discovermagazine.com/planet-earth/why-are-there-seven-days-in-a-week Why Are There Seven Days in a Week?]. ''Discover'' (15 January 2020). Retrieved 2022-10-22.</ref> This later spread across Europe, then the rest of the world.

[[File:First Day of Week World Map.svg|thumb|350px|World map showing the first day of the week used in different countries according to the [[Common Locale Data Repository]]<ref>{{Cite web |title=Territory Information |url=https://www.unicode.org/cldr/charts/45/supplemental/territory_information.html |access-date=12 July 2024 |website=www.unicode.org}}</ref>{{legend|#fdc086|Monday}} {{legend|#f0027f|Friday}} {{legend|#6fa96f|Saturday}} {{legend|#386cb0|Sunday}}]]

In English, the [[names of the days of the week]] are [[Sunday]], [[Monday]], [[Tuesday]], [[Wednesday]], [[Thursday]], [[Friday]], and [[Saturday]]. In many languages, including English, the days of the week are named after gods or classical planets. Saturday has kept its Roman name, while the other six days use Germanic equivalents. Such a week may be called a ''planetary week'' (i.e., a classical planetary week).<ref>{{Cite book|last=Lagasse|first=Paul|url=https://search.credoreference.com/content/entry/columency/week/|title=The Columbia Encyclopedia|publisher=Columbia University Press|year=2018|chapter=Week}}</ref> Certain weeks within a [[year]] may be designated for a particular purpose, such as [[Golden Week (China)|Golden Week in China]] and [[Golden Week (Japan)|Japan]], and [[National Family Week]] in Canada. More informally, certain groups may advocate [[awareness week]]s, which are designed to draw attention to a certain subject or cause. The term "week" may also be used to refer to a sub-section of the week, such as the [[workweek and weekend]].

Cultures vary in which days of the week are designated the first and the last, though virtually all have Saturday, Sunday or Monday as the first day. The [[Geneva]]-based ISO standards organization uses Monday as the first day of the week in its [[ISO week date]] system through the international [[ISO 8601]] standard.{{efn|"ISO 8601 Data elements and interchange formats – Information interchange – Representation of dates and times" is an international standard covering the exchange of date- and time-related data.}} Most of Europe and China consider Monday the first day of the (work) week, while North America, Israel, South Asia, and many Catholic and Protestant countries, consider Sunday the first day of the week. Saturday is judged as the first day of the week in much of the [[Middle East]] and [[North Africa]] due to the Islamic influence.{{citation needed|date=July 2024}} Other regions are mixed, but typically observe either Sunday or Monday as the first day.<ref name="firstday">{{Cite web |title=Territory Information |url=https://unicode-org.github.io/cldr-staging/charts/38/supplemental/territory_information.html |access-date=6 November 2020 |website=www.unicode.org}}</ref>

The three [[Abrahamic religions]] observe different days of the week as their holy day. [[Jews]] observe their [[Sabbath]] ([[Shabbat]]) on Saturday, the seventh day, from sundown Friday to sundown Saturday, in honor of God's [[Genesis creation narrative|creation]] of the world in six days and then resting on the seventh. Most [[Christians]] observe Sunday (the [[Lord's Day]]), the first day of the week in traditional Christian calendars, in honor of the [[resurrection of Jesus]]. [[Muslims]] observe their [[Sabbath#Islam|"day of congregation"]], known as {{transl|ar|yaum al-[[jumu'ah|jum`ah]]}}, on Friday because it was described as a sacred day of congregational worship in the [[Quran]].<ref>{{cite web |last1=Aslan |first1=Rose |title=What is the significance of Friday prayers in Islam? |url=https://theconversation.com/what-is-the-significance-of-friday-prayers-in-islam-113702 |website=The Conversation |date=19 March 2019 |access-date=10 March 2023}}</ref>