Week

A week (in the Western and international context) is a unit of time equal to seven days. It is the standard calendrical period between a day and a month in most parts of the world. There are just over 52 weeks in a year, or on average 4+1⁄3 weeks in a month. The days of the week are often used to set work days, rest days, and holy days. The term "week" is also used to refer to a sub-section of the week, such as the workweek and weekend. Certain weeks within a year may be designated for a particular purpose, such as Golden Week in China and Japan. More informally, certain groups may advocate awareness weeks, such as National Family Week in Canada, which are designed to draw attention to a certain subject or cause. Some ancient and traditional cultures have or had different week lengths, including ten days in Egypt, an eight-day week for Etruscans and ancient Romans, and four- or five-day weeks between market days in modern West Africa. (See also the Japanese ten-day jun, used alongside the 7-day week.) The Romans later switched to a seven-day week. This was originally a so-called planetary or astrological week, a cycle of disputed origins that named each day after a classical planet and the god identified with that planet. Due to the spread of Christianity, the Romans' astrological week was identified and merged with the Christian seven-day week centred on Sunday (the day of the Resurrection of Christ), which had originated with the Jewish seven-day week centred on Saturday. Both the astrological week and the Jewish week may have been inspired by seven-day cycles in the Babylonian calendar, although the evidence for that is weak. In AD 321, Emperor Constantine the Great decreed a seven-day week in the Roman Empire, including making Sunday a public holiday. This spread across Europe, then the rest of the world.

In English, the days use Germanic equivalents of the Roman names, apart from Saturday, which had no equivalent: Sunday (Sol/sun day), Monday (Luna/moon day), Tuesday (Mars/Tiw's day), Wednesday (Mercury/Woden's day), Thursday (Jupiter/Thor's day), Friday (Venus/Friga's day), and Saturday (Saturn day). Which days of the week are designated the first and the last varies by culture, though virtually all have Saturday, Sunday, or Monday as the first day. The three Abrahamic religions observe different days of the week as their holy day. Christians observe the Sabbath on Sunday (see Sabbath in Christianity), traditionally the first day of the week, in honor of the resurrection of Jesus. Muslims observe the Sabbath on Friday (see Friday prayer), because it was described as a sacred day of congregational worship in the Quran. Jews observe the Sabbath on Saturday (Shabbat), the seventh day, in honor of God's creation of the world in six days and resting on the seventh.

Name The English word week comes from the Old English wice, ultimately from a Common Germanic *wikōn-, from a root *wik- "turn, move, change". The Germanic word probably had a wider meaning before the Roman calendar was adopted, perhaps "succession series", as suggested by Gothic wikō translating taxis "order" in Luke 1:8. The seven-day week is named in many languages by a word derived from "seven". The archaism sennight ("seven-night") preserves the old Germanic practice of reckoning time by nights, as in the more common fortnight ("fourteen-night"). Hebdomad and hebdomadal week both derive from the Greek hebdomás (ἑβδομάς, "a seven"). Septimana is cognate with the Romance terms derived from Latin septimana ("seven mornings").

Definition and duration A week is defined as an interval of exactly seven days, so that, except when passing through daylight saving time transitions or leap seconds,

1 week = 7 days = 168 hours = 10,080 minutes = 604,800 seconds. With respect to the Gregorian calendar:

1 Gregorian calendar year = 52 weeks + 1 day (2 days in a leap year) 1 week = 1600⁄6957 ≈ 22.9984% of an average Gregorian month In a Gregorian mean year, there are 365.2425 days, and thus exactly 52+71⁄400 or 52.1775 weeks (unlike the Julian year of 365.25 days or 52+5⁄28 ≈ 52.1786 weeks). There are exactly 20,871 weeks in 400 Gregorian years, so 27 August 1626 was a Thursday, just as was 27 August 2026. Relative to the path of the Moon, a week is 23.659% of an average lunation or 94.637% of an average quarter lunation. Historically, the system of dominical letters (letters A to G identifying the weekday of the first day of a given year) has been used to facilitate calculation of the day of week. The day of the week can be easily calculated given a date's Julian day number (JD, i.e. the integer value at noon UT): Adding one to the remainder after dividing the Julian day number by seven (JD modulo 7 + 1) yields that date's ISO 8601 day of the week. For example, the Julian day number of 27 August 2026 is 2461280. Calculating 2461280 mod 7 + 1 yields 4, corresponding to Thursday. In 1973, John Conway devised the Doomsday rule for mental calculation of the weekday of any date in any year.

Start and end

Cultures vary as to which days of the week are designated the first and the last on their calendars, though virtually all have Saturday, Sunday, or Monday as the first day. The International Organization for Standardization uses Monday as the first day of the week in its ISO week date system through the international ISO 8601 standard. Most of Europe, China, West and Central Africa, and Oceania count Monday as the first day of the week, while much of America, South and Southeast Asia, and southern Africa count Sunday as the first. Most Arabic- and Persian-speaking countries use Saturday as the first day of the week, though some use Monday (Lebanon, Morocco, Tunisia and Tajikistan) or Sunday (Saudi Arabia and Yemen). Other regions are mixed, but nearly all observe either Sunday or Monday as the first day. The first day used in calendars may differ from the numbered "name" of the day in a particular place.

Days of the week

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