Week

A week is a period of seven days, and although it does not rely on any astronomical basis, it is widely used as a unit of time. The week can be thought of as an independent calendar running in parallel with various other calendars. However, some calendars make the week dependent by having days that do not belong to the week as in the World calendar or in the French Revolutionary Calendar, which had weeks of 10 days.

The origin of a seven-day period is generally associated with the ancient Jews and the biblical account of the creation, according to which God laboured for six days and rested on the seventh. However, the ancient Babylonians were known to have observed a fixed seven-day week before the Jews adopted the idea. The Babylonian use of the seven-day week eventually influenced other cultures in Eastern Mediterranean and the Middle East. The use of the fixed 7-day period was probably a simplification of a part of a lunar month. Meanwhile both the Babylonians and the Jews retained the lunar calendar while using the 7-day week. There are 7 primary heavenly bodies: the sun, moon and five planets visible to the naked eye, and this may explain the seven-day week.

Various groups of citizens of the Roman Empire adopted the week, especially those who had spent time in the eastern parts of the empire including Egypt where the 7-day week was in use. Contemporaneously, Christians picked up the practice from the Jews and spread the week's use along with their religion.

As the early Christians evolved from being Jewish to being a distinct group, various groups evolved from celebrating both the Jewish Sabbath (Saturday) and the first Day or the Lord's Day (Sunday), to only celebrating Sunday.

In the early 4th century (CE), the Roman Emperor Constantine regulated the use of the week due to a problem of the myriad uses of various days for religious observance, and established the first day as the day for religious observance for all groups, not just those Christians and others who were already observing Sunday. The Jews retained their (at least) 800-year-old tradition of Saturday observance. Later, after the establishment of Islam, Friday became that religion's day of observance.

The 7-day-week concept defined in the ancient Middle East is now used in most of the world due to the spread of modern commercial trade and business. China adopted the concept of the week only in modern times when the western calendar system was introduced to China. The word "week" is translated as "planet period" in Chinese, possibly based on some misunderstanding of the translators when the concept was first introduced to China. In some archaic Chinese references, the days of the week were named after the Sun (日曜日 Sunday), the moon, and the five major planets, Mars, Mercury, Jupiter, Venus and Saturn (土曜日 Saturday) in that order. The Japanese language still preserved the same naming of the week though the Chinese no longer uses it. In modern Chinese, the day of week are named by number, e.g. Monday is called "planet period one" (星期一) etc.

Days of the week:

In English the names of the days mostly come from Saxon gods and goddesses:

• Monday - Moon
• Tuesday - Tyr
• Wednesday - Wodan or Odin
• Thursday - Thor
• Friday - Freyr
• Saturday - Saturn
• Sunday - the Sun

Depending on how these gods and goddesses were associated with the planets in Saxon astrology, it may explain why the word "week" was translated to "planet period" in the Far East. If Tyr, Wodan, Thor and Freyr were associated with the planets Mars, Mercury, Jupiter and Venus respectively, then the Chinese and Japanese translation were consistent with the origin of the names.

Saturday and Sunday are commonly called the weekend and are days of rest and recreation in most western cultures.

According to the ISO 8601 norm the week begins on a Monday. This corresponds with the term weekend for the Saturday and Sunday. However following Constantine's decision to make the first day of the week the day of religious observance, Sunday may also be considered the first day of the week in historically Christian countries. In this regard calendars exist in two varieties.

The Thursdays of a year determine the week numbering: week 1 is defined as the week that contains the first Thursday of the year, etc., see also ISO 8601.

Facts and Figures:

• 1 week = 7 days
• 1 week = 168 hours = 10,080 minutes = 604,800 seconds
• 1 year = 52 weeks + 1 day (2 days in a leap year)
• 1 week = 23% of an average month (almost exactly)

In a Gregorian mean year there are exactly 365.2425 days, and thus exactly 52.1775 weeks. There are exactly 20871 weeks in 400 Gregorian years, so 25 December 1601 was a Tuesday just like 25 December 2001.

see also calendar, times from 100 kiloseconds to 1 megasecond