"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. Such a week may be called a planetary week. This is in contrast to the lunar week in the Bible in which the seven days are simply numbered with seventh day being kept as a sabbath day.
While, for example, the United States, Canada and Japan 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 has Monday as the first day of the week. The ISO standard includes the ISO week date system, a numbering system for weeks within a given year, where each week beginning on a Monday is associated with the year that contains that week´s Thursday (so that if a year starts in a long weekend Friday–Sunday, week number one of the year will start after that). ISO 8601 assigns numbers to the days of the week, running from 1 to 7 for Monday through to Sunday." - (en.wikipedia.org 17.12.2019)
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