A week is a time unit equal to seven days. It is the standard time period used for cycles of rest days in most parts of the world, mostly alongside—although not strictly part of—the Gregorian calendar.
In many languages, the days of the week are named after classical planets or gods of a pantheon. In English, the names are Monday, Tuesday, Wednesday, Thursday, Friday, Saturday and Sunday, then returning to Monday. Such a week may be called a planetary week. This arrangement is similar to a week in the New Testament in which the seven days are simply numbered with the first day being a Christian day of worship (aligned with Sunday, offset from ISO 8601 by one day) and the seventh day being a sabbath day (Saturday).
While, for example, the United States, Canada, Brazil, Japan and other countries consider Sunday as the first day of the week, and while the week begins with Saturday in much of the Middle East, the international ISO 8601 standard and most of Europe has Monday as the first day of the week. The Geneva-based ISO standards organization uses Monday as the first day of the week in its ISO week date system.
The term "week" is sometimes expanded to refer to other time units comprising a few days, such as the nundinal cycle of the ancient Roman calendar, the "work week", or "school week" referring only to the days spent on those activities.