A week is a unit of time equal to seven days. It is the standard time period used for short cycles of days in most parts of the world. The days are often used to indicate common work days and rest days, as well as days of worship. Weeks are often mapped against yearly calendars. There are just over 52 weeks in a year. The term "week" may also be used to refer to a sub-section of the week, such as the workweek and weekend.

Ancient cultures had different "week" lengths, including ten days in Egypt and an eight-day week for Etruscans. The Etruscan week was adopted by the 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 AD 321, Emperor Constantine the Great officially decreed a seven-day week in the Roman Empire, including making Sunday a public holiday. This later spread across Europe, then the rest of the world.

In English, the names of the days of the week are Monday, Tuesday, Wednesday, Thursday, Friday, Saturday and Sunday. 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). 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.

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. Most of Europe and China consider Monday the first day of the (work) week, while North America, South Asia, and many Catholic and Protestant countries, consider Sunday the first day of the week. It is also the first day of the week in almost all of the Arabic speaking countries.^{[contradictory]} This is culturally and historically the case since in Arabic Sunday is referred to as "Yaom Al'Ahad" which literally means "The first day". Other regions are mixed, but typically observe either Sunday or Monday as the first day.

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 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 "day of congregation", known as yaum al-jum`ah, on Friday because it was described as a sacred day of congregational worship in the Quran.

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 prior to the adoption of the Roman calendar, 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").

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