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A week is a unit of time longer than a day and shorter than a month. In most modern calendars, including the Gregorian Calendar, the week is a period of seven days, making it the longest conventionally used time unit that contains a fixed number of days. Although having no direct astronomical basis, it is widely used as a unit of time. Weeks can be thought of as forming an independent continuous calendar running in parallel with various other calendars. However, some calendars have been designed so that a given date occurs on the same day of the week each year. This can be done by making the week dependent on the year, with some days each year that do not belong to any week: the proposed World Calendar has 52 weeks and 1 or 2 extra days each year, while the French Revolutionary Calendar had 36 weeks of 10 days and 5 or 6 extra days. The year can also be made dependent on the week: the former Icelandic calendar had years of 52 or 53 weeks.

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. The Babylonian use of the seven-day week eventually influenced other cultures of the Eastern Mediterranean and the Middle East. The fixed 7-day period was probably a simplification of 1/4 of a lunar month. Meanwhile both the Babylonians and the Jews retained the lunar calendar while using the 7-day week. There are 7 heavenly bodies or "luminaries" normally visible to the naked eye (the Sun, Moon, and 5 visible planets), and this explained the 7-day week, both scientifically and astrologically.

Various groups of citizens of the Roman Empire adopted the week, especially those who had spent time in the eastern parts of the empire, such as 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 Sunday as the day for religious observance and rest 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, originally a Babylonian or Sumerian invention, soon became a practice among Christians, Jews, and Muslims. Following European colonization and the subsequent rise of global corporate business, the 7-day week has become universal in keeping time, even in cultures that did not practise it before.

Table of contents
1 China, Japan and India
2 Days of the Week
3 The Order of the Days of the Week
4 Facts and Figures
5 External links

China, Japan and India

China adopted the concept of the work week only in modern times when the Western calendar system was introduced to China in 1911. There are multiple terms for "week" in Chinese. The best-known to Westerners is 星期 (Xing1 Qi2 or "Star Period"). According to this site:

In Second Century China, a method of recording time was invented, called the 七曜曆 (Qi1 Yao4 Li4 or "Seven Luminaries Calendar"), but it did not definitely contain the Seven Luminaries method of counting days. In the Eighth Century, Manichaeism travelled from 康居國 (Kang1 Ju1 Guo2; Cossack Country or Khazar Country?) carrying the Seven Luminaries method of counting days and transmitted it into China.

Slightly earlier, the passage explained the nomenclature of the adopted system:

The Seven Luminaries were used to count the days at some ancient date. This other method began in ancient Babylon, one of seven days comprising a week, the Sun Luminary, the Moon Luminary, the Fire Luminary (Mars), the Water Luminary (Mercury), the Wood Luminary (Jupiter), the Metal Luminary (Venus), and the Land Luminary (Saturn), respectively, comprised the original system and were called the "星期" (Xing1 Qi2 or "Star Period").

In the early Chinese system, the days of the week were named after the Sun (日曜日 Sunday), the moon (月曜日 Monday), and the five major planets, Mars (火曜日 Tuesday), Mercury (水曜日 Wednesday), Jupiter (木曜日 Thursday), Venus (金曜日 Friday) and Saturn (土曜日 Saturday) in that order. The Japanese language still preserves the same naming of the week, though the Chinese no longer use it. In modern Chinese, the days of the week are numbered, e.g. Monday is called "planet period one" (星期一) etc. It is interesting to note that Sunday still retains the "sun" in its name, most likely due to the Christian Sabbath that became the Chinese day of rest.

Indian calendars (some lunar and some solar) also have the names of the days named after the luminaries in the same order as Chinese.

The final term for "week" in Chinese is 禮拜 (Li3 Bai4), which means something like "Prayer Ritual". This was almost certainly introduced by Christian missionaries who would have wanted Chinese people to pray weekly on the previously mentioned Sabbath, as the Chinese Ancestor Religion requires daily or calendrically defined offerings and has no "day of rest".

Days of the Week

In English the names of the days mostly come from Norse gods and goddesses (Saturday being the only one named after a Roman god). In most Romance languages (Spanish for example in the table below), the names of the days come from Roman gods, and have corresponding astrological symbols:

English Name Spanish Name Norse Association Roman Association Astrological Symbol
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Monday Lunes Moon Day
Tuesday Martes Tyr's Day The Day of Mars
Wednesday Miercoles Wodan or Odin's Day The Day of Mercury
Thursday Jueves Thor's Day The Day of Jupiter
Friday Viernes Freyr or Freya's Day The Day of Venus
Saturday Sabado Saturn's Day The Day of Saturn
Sunday Domingo Sun Day

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

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 traditional Sunday-first system is used by most English-speakers, while most of continental Europe uses the ISO order.

ISO 8601 also includes a numbering system for weeks; each week is associated with the year in which Thursday occurs. Thus, for example, week 1 of 2004 ran from Monday, December 29, 2003, to Sunday, January 4, 2004; the highest week number in a year may be 52 or 53. This style of numbering is commonly used (for example, by businesses) in some European countries, but rare elsewhere.

A system of Dominical letters has been used to determine the day of week in the Gregorian or the Julian calendar.

The Order of the Days of the Week

The order of the days of the week was explained by Dio Cassius (and Chaucer gave the same explanation in his Treatise on the Astrolabe). According to Cassius, it was a principle of astrology that the heavenly bodies presided, in succession, over the hours of the day. The Ptolemaic system asserts that the order of the heavenly bodies, from the farthest to the closest to the earth, are: Saturn, Jupiter, Mars, Sun, Venus, Mercury, Moon. If the first hour of a day is dominated by Saturn, then the second hour is dominated by Jupiter, the third by Mars, and so on, so that the sequence of planets repeats every seven hours. Therefore, the twenty-fifth hour, which is the first hour of the following day, is dominated by the Sun; the forty-ninth hour, which is the first hour of the next day, by the Moon. Thus, if a day is labelled by the planet which dominates its first hour, then Saturn's day is followed by the Sun's day, which is followed by the Moon's day, and so forth.

Facts and Figures

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, calculating the day of the week

External links