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, but are typically not the basis for them, as weeks are not based on astronomy.

The modern seven-day week can be traced back to the Babylonians, who used it within their calendar. Other ancient cultures had different week lengths, including ten 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. In 321 CE, Emperor Constantine 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, the days of the week are named after gods or planets visible to the eye. Such a week may be called a planetary week. Certain weeks within a year may be designated for a particular purpose, such as Holy Week in Christianity, Golden Week in China, and National Family Week in Canada. More informally, certain groups may advocate awareness weeks, 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. Most of Europe and China consider Monday the first day of the week, most of North America and South Asia consider Sunday the first day, while Saturday is judged as the first day of the week in much of the Middle East and North Africa. Other regions are mixed, but typically observe either Sunday or Monday as the first day. The Jewish week ends with nightfall on Saturday, at the conclusion of the Sabbath, following the Hebrew Bible in which God created the world in six days and rested on the seventh. Christians set their Sabbath to Sunday, so as not to coincide with Judaism. Muslims set their Sabbath to Friday because it was described as a sacred day of worship in the Quran.