Seven-day week
A week is a time unit equal to seven days. It is the standard time period used for cycles of work days and rest days in most parts of the world, mostly alongside (but not strictly part of) the Gregorian calendar.
The days of the week were named after the classical planets (derived from the astrological system of planetary hours) in the Roman era. In English, the names are Sunday, Monday, Tuesday, Wednesday, Thursday, Friday, and Saturday.
ISO 8601 includes the ISO week date system, a numbering system for weeks within a given year – each week begins on a Monday and 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.
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, or the "work week" or "school week" referring only to the days spent on those activities.
A week is defined as an interval of exactly seven days, so that technically, except at daylight saving time transitions or leap seconds,
With respect to the Gregorian calendar:
In a Gregorian mean year, there are 365.2425 days, and thus exactly 52 71⁄400 or 52.1775 weeks (unlike the Julian year of 365.25 days or 52 5⁄28 ≈ 52.1786 weeks, which cannot be represented by a finite decimal expansion). There are exactly 20,871 weeks in 400 Gregorian years, so 25 January 1617 was a Wednesday just like 25 January 2017.
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