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. Occasionally, this arrangement is instead 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). This is based on the Jewish week as reflected in the Hebrew Bible (also appears as the Old Testament in the Christian Bible). The Hebrew Bible offers the explanation that God created the world in six days. The first day is then given the literal name First (in Hebrew: ראשון), the second being called Second (שני) and so forth for the first six days, with the exception of the seventh and final day, which rather than be called Seventh (שביעי), is called Shabbat (שבת) from the word לשבות (to rest). The biblical text states this is because that was the day when God rested from his work of creating the world. Shabbat (equivalent to Saturday) therefore became the day of worship and rest in Jewish tradition and the last day of the week, while the following day, Sunday, is the first one in the Hebrew week. Thousands of years later, these names are still the names of the weekdays in Hebrew, and this week construct is still the one observed in Jewish tradition.
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.
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