The weekdays and weekend are the complementary parts of the week, devoted to labour and rest, respectively. The legal weekdays (British English), or workweek (American English), is the part of the seven-day week devoted to working. In most of the world, the workweek is from Monday to Friday and the weekend is Saturday and Sunday. A weekday or workday is any day of the working week. Other institutions often follow this pattern, such as places of education. The constituted weekend has varying definitions, based on determined calendar days, designated period of time, and/or regional definition of the working week (e.g., commencing after 5:00 p.m. on Friday and lasting until 6:00 p.m. on Sunday). Sometimes the term "weekend" is expanded to include the time after work hours on the last workday of the week. Weekdays and workdays can be further detailed in terms of working time, the period of time that an individual spends at paid occupational labor.

In many Christian traditions, Sunday is the "day of rest and worship". The Jewish Shabbat or Biblical Sabbath lasts from sunset on Friday to the fall of full darkness on Saturday; as a result, the weekend in Israel is observed on Friday to Saturday. Some Muslim-majority countries historically instituted a Thursday–Friday weekend. Today, many of these countries, in the interests of furthering business trade and cooperation, have shifted to Friday–Saturday or Saturday–Sunday.

The Christian day of worship is just one day each week, but the preceding day (the Jewish Sabbath) came to be taken as a holiday as well in the 20th century. This shift has been accompanied by a reduction in the total number of hours worked per week. The present-day concept of the "weekend" first arose in the industrial north of Britain in the early 19th century. A day off is a non-working day, not necessarily on weekends.

Some countries have adopted a six-day workweek and one-day weekend (6×1), which can be Friday only (in Djibouti, Iran, Somalia and Libya), Saturday only (in Nepal), or Sunday only (in Mexico, Colombia, Uganda, Eritrea, India, Philippines, and Equatorial Guinea). However, most countries have adopted a five-day workweek and two-day weekend (5×2), whose days differ according to religious tradition: Friday and Saturday (in 17 Muslim countries and Israel); Saturday and Sunday (most of the countries); or Friday and Sunday (in Brunei Darussalam, Aceh (Indonesia) and Sarawak (Malaysia)), with the previous evening post-work often considered part of the weekend. Proposals continue to be put forward to reduce the number of days or hours worked per week, such as the four-day workweek, on the basis of predicted social and economic benefits.

A continuous seven-day cycle that runs throughout history, paying no attention whatsoever to the phases of the moon and having a fixed day of rest, was most likely first practised in Judaism, dated to the 6th century BC at the latest.

In Ancient Rome (753 BC–476 AD), every eight days there was a nundinae. It was a market day, during which children were exempted from school and agricultural workers stopped work in the field and came to the city to sell the produce of their labor or to practice religious rites.^{[citation needed]}

The French Revolutionary Calendar (1793–1805) had ten-day weeks (called décades) and allowed décadi, one out of the 10 days, as a leisure day. From 1929 to 1940, the Soviet Union utilized a calendar with five and six-day work weeks, with a rest day assigned to a worker either with a colour or number.^{[citation needed]}

During the Han dynasty of imperial China, officials had a day off once every five days known as hsui-mu （休沐）. This rest day was known as "a day for rest and for washing one's hair".