The origin of the concept of the week may be attributed to Genesis 1 and 2 in the Bible, which contains the creation account of the earth, animals, and man in six days, with the sabbath of rest on the seventh day (Genesis 2:2-4). Its existence in ancient cultures may be traced to a divine or spiritual origin for this institution, since there is no celestial explanation to account for it, such as exists with the day, the month, and the year.
Various sources point to the seven day week as having originated in ancient Babylonia or Sumer. It has been suggested that a seven day week might be much older. The seven day planetary week originated in Hellenistic Egypt.
The 1943 Universal Jewish Encyclopedia volume 10 page 482, edited by Isaac Landman under the article “Week”, written by Simon Cohen, Director of Research, summarizes:
Meanwhile, the Roman Republic and then Empire, like the Etruscans, used a "market week" of eight days (known as the nundinal cycle). From around the 1st century CE, with the spread of Christianity, the Roman eight day week was replaced gradually by the seven day week.
The seven day weekly cycle is known to have remained unbroken in Europe for almost two millennia despite changes to the Alexandrian, Julian, and Gregorian calendars. The date of Easter Sunday can be traced back through numerous computistic tables to an Ethiopic copy of an early Alexandrian table beginning with the Easter of 311 CE as described by Otto Neugebauer in Ethiopic astronomy and computus. Only one Roman date with an associated day of the week exists from the first century and it agrees with the modern sequence, if properly interpreted. Jewish dates with a day of the week do not survive from this early period. In the case of the Jewish week, it had been in use for at least 1,000 years before its adoption by the Roman Empire.
Other theories speculate that the fixed seven-day period appeared due to evenly dividing a lunar month into quarters.
The meaning of the name of the days of the week in Sanskrit, above, is equivalent to English names: e.g., "Shani-Vaar," where Shani = Saturn and Vaar = Day.
The earliest known reference in Chinese writings is attributed to Fan Ning, who lived in the late 4th century, while diffusions via India are documented with the writings of the Chinese Buddhist monk Yi Jing and the Ceylonese or Central Asian Buddhist monk Bu Kong of the 7th century. The Chinese transliteration of the planetary system was soon brought to Japan by the Japanese monk Kobo Daishi; surviving diaries of the Japanese statesman Fujiwara Michinaga show the seven day system in use in Heian Period Japan as early as 1007. In Japan, the seven day system was kept in use (for astrological purposes) until its promotion to a full-fledged (Western-style) calendrical basis during the Meiji era.
The Chinese use of the seven day week (and thus Korean, Japanese, Tibetan, and Vietnamese use) traces back to the 600s CE. The 28 stars were arranged in order of sun, moon, fire, water, wood, gold, earth, and every 7 days were called "qi-yao". The days were assigned to each of the luminaries, but the week did not affect social life or the official calendar. The law in the Han Dynasty required officials of the empire to rest every 5 days, called "mu", while it was changed into 10 days in the Tang Dynasty, called "huan" or xún (旬). With months being almost 3 weeks long (alternating 29 and 30 days) the weeks were labelled shàng xún (上旬), zhōng xún (中旬), and xià xún (下旬) which mean roughly "upper", "middle" and "lower" week. The 7 days "week" in ancient China is mostly kept in astrological purposes and cited in several Buddhist texts until the Jesuits reintroduced the concept in the 16th century. Thus the 19th century Japanese, when adopting the seven day western week, took their own astrological week with names for the days of the week that corresponded to the English names (and in fact were better preservations of the original Babylonian concepts, the English day names having been conflated with gods from Germanic mythology).
The weekly cycle runs concurrently with regular calendar cycles. The weekly cycle is not based on any astronomical phenomena. Besides being of religious significance, it also is convenient in commercial and social contexts. Some novel calendars have been designed which synchronise the weeks and years by adding a leap week or weekless days to the calendar. The advantage of these calendar systems is that each year a given date always falls on the same day of the week. For example, the proposed World Calendar has 52 weeks and one or two extra weekless days each year, which do not count in the weekly cycles. The short-lived 18th century French Revolutionary Calendar had 36 weeks of 10 days and five or six extra weekless days. On the other hand, the former Icelandic calendar had years of 52 or 53 weeks. Instead of adding extra weekless days, the number of weeks in the calendar year varied. An early Norse calendar, from the beginning of the Viking Age, had five day weeks, called fimmts, arranged in 12 months of six fimmts each, with five ceremonial days not part of any month. The Hermetic Lunar Week Calendar uses the lunar week which is a quarter of a lunation and has 6, 7, 8 or 9 days (average 7.382647 days).
In Jewish and Christian tradition, the first day of the week is Sunday. Both the Hebrew and Ecclesiastical Latin languages number most of the days of the week. In Hebrew, Sunday through Friday are numbered one through six, while in Ecclesiastical Latin, Monday through Friday are numbered the second through the sixth days of the week (feria). For Christians and Jews, Sunday remains the first day of the week. Most, though not all, business and social calendars in North America mark Sunday as the first day of the week.
In most of Europe, and some other countries, Monday is considered to be the first day of the week and is literally named as such in languages such as Mandarin (pinyin Xīngqí Yī, literally means Weekday One) and Lithuanian (pirmadienis - means First Day).
The seven-day workweek is generally composed of five working days ("weekdays") and two non-working days (the "weekend"), though which days of the week are which varies from country to country. Which day of the week is the "first" day also varies, even among countries that share the same weekend days.
In a Gregorian mean year there are exactly 365.2425 days, and thus exactly 52.1775 weeks (unlike the Julian year of 365.25 days, which does not contain a number of weeks represented by a finite decimal expansion). There are exactly 20871 weeks in 400 Gregorian years, so 10 April 1605 was a Sunday just like 10 April 2005.
ISO 8601 includes the ISO week date system, a numbering system for weeks – 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). For example, week 1 of 2004 (2004W01) ran from Monday 29 December 2003 to Sunday, 4 January 2004, because its Thursday was 1 January 2004, whereas week 1 of 2005 (2005W01) ran from Monday 3 January 2005 to Sunday 9 January 2005, because its Thursday was 6 January 2005 and so the first Thursday of 2005. The highest week number in a year may be 52 or 53 (it was 53 for year 2004).
The numbering system in different countries may deviate from the international ISO standard. There are at least six possibilities:
|First day of week||First week of year contains||Weeks assigned twice||Used by/in|
|Monday||1 January,||1st Sunday,||1–7 days of year||yes||UK|
|Monday||4 January,||1st Thursday,||4–7 days of year||no||Most of Europe ISO 8601(1988), European Norm EN 28601 (1992)|
|Monday||7 January,||1st Monday,||7 days of year||no|
|Wednesday||1 January,||1st Tuesday,||1–7 days of year||yes|
|Saturday||1 January,||1st Friday,||1–7 days of year||yes|
|Monday||1 January,||1st Saturday,||1–7 days of year||yes||USA|
Although this simple primitive conception gave place in time, as feasts were introduced and multiplied, to an annual calendar, the week always retained its importance; this is particularly seen in the Divine Office in the hebdomadal division of the Psalter for recitation. Amalarius preserves for us the particulars of the arrangement accepted in the chapel royal at Aachen in 802 CE by which the whole Psalter was recited in the course of each week. In its broader features the division was identical with that theoretically imposed by the Roman Breviary until the recent publication of the Apostolic Constitution "Divine afflatu" on 1 Nov., 1911 CE. Moreover, it appears from Amalarius that the Carlovingian arrangement was in substance the same as that already accepted by the Roman Church. Already in the sixth century, St. Benedict had clearly laid down the principle that the entire Psalter was to be recited at least once in the week; indeed a similar arrangement was attributed to Pope St. Damasus. The consecration of particular days of the week to particular subjects of devotion is also officially recognized by the special Office of the Blessed Virgin on the Saturday, by the Friday Masses of the Passion during Lent and by the arrangement of Votive Offices for special week days approved by Pope Leo XIII. For a long time in the early Middle Ages, Thursday was regarded in the West as a sort of lesser feast or Sunday, probably because it was the day of the week on which the Ascension fell (cf. Bede, Historia ecclesiastica gentis Anglorum, IV, 25). Again the Breviary approved after the Council of Trent left certain devotion accretions to the Office, e.g. the Office for the Dead, Gradual Psalms, etc, to be said once a week, particularly on the Mondays of Advent and Lent.