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.

The week as indicator of market day

Although seven day weeks are common to all modern societies now, anthropologists note that weeks of other durations (varying from three to eight days) are found in many pre-modern societies. They also observe that the name for "week" is often the same as that for "market day", suggesting the concept of a week is likely to arise in any agrarian or pre-agrarian society where people have marketplaces or market days. In sparsely populated areas where trade is not conducted every day it is essential that farmers and consumers agree in advance on what day they will meet, especially if the walk to market takes several hours or days. The week (meaning a fixed count of days) was much simpler and more precise way of doing this when compared with a lunar calendar-based system or a system based on the seasonal rotation of the celestial sphere. Being based on a count kept by people rather than on the relative motion of the moon and stars, the week was not "heavenly", but in the traditional seven-day week, this was overcome by assigning the sun, moon, and the five planets known to the ancients (Mercury, Venus, Mars, Jupiter, Saturn) each to a specific day of the week.

Origin of the seven-day week

Enlarge picture
Weekday heptagram used for the planets or the days of the week
The seven-day week became established in both the West and East according to different paths:

Hindu, Babylonian, and Jewish seven-day week

  • Hindu civilization employed a seven-day week, mentioned in the Ramayana, a sacred epic written in Sanskrit about 500 BCE, as Bhanu-vaar meaning Sunday, Soma-vaar meaning Moon-day and so forth.
  • The ancient Babylonians observed a seven-day week, stemming from astronomical observation and association. Days and deities were based on the seven heavenly bodies or "luminaries" visible to the naked eye (the Sun, Moon, and 5 visible planets).
  • The Hebrew (and later Christian, and Muslim) seven-day week corresponds to the biblical creation story, in which God created the universe in six days, then rested on the seventh.
Other theories speculate that the fixed seven-day period appeared due to evenly dividing a lunar month into quarters.

Chinese seven-day week

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). By contrast, the Japanese names refer to the Chinese Sun, Moon and the five planets. The only difference is that the planets in the Japanese week have Chinese names based on the five elements rather than pagan deities.[1]

Later use of the 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, such as Egypt, where the 7-day week was in use. Contemporaneously, Christians, following the biblical instruction, 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 celebrating only Sunday.

In 321 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 of the 4th century retained their tradition of Saturday observance, by then 800 to 1700 years old, and continue to do so. Later, after the establishment of Islam, Friday became that religion's day of observance.

The seven-day week soon became a practice among Christians, Jews, and Muslims. Following European colonization and the subsequent rise of global corporate business, the seven-day week has become universal in keeping time, even in cultures that did not practise it before. Because of the two-day weekend, some modern calendars end the week on Sunday and begin it on Monday. The ISO week date, part of the international standard ISO 8601, also defines Monday as the first day of the week. In practice, this means that calendar formats disagree, and that "next week" said on Sunday means "the week beginning tomorrow".

In that international standard, the "first week of the year" is that week which includes the first Thursday of the year. This way, if a year starts in a long weekend Friday–Sunday, week 1 of the year will start after that. Since the New Year's Day itself is a holiday in many countries, this means that the first working day of the year is in week 1.

Weeks and the calendar year

Although without a direct astronomical basis (seven days is just under a quarter of a lunar month), it is widely used as a unit of time, especially in the social and commercial context. Weeks can be thought of as forming an independent continuous calendar running in parallel with various other calendars.

However, some novel calendars have been designed in which the weeks and years are forced into synchronization by adding a leap week or weekless days into the calendar. The advantage of these calendars is that a given date always falls on the same day of the week every year. For example the proposed World Calendar has 52 weeks and one or two extra days each year, while the 18th century French Revolutionary Calendar had 36 weeks of 10 days and five or six extra days. Alternatively, instead of adding extra days outside of weeks, it is possible to add entire weeks to the calendar if the years are allowed to vary in length by more than a day; for example, the former Icelandic calendar had years of 52 or 53 weeks. 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).

Days of the week

In English the days of the week are Monday, Tuesday, Wednesday, Thursday, Friday, Saturday and Sunday.

Monday is considered in many cultures to be the first day of the week and is literally named as such in languages such as Mandarin ('xinqiyi') and Lithuanian ('pirmadienis'). Even though the holiday day of rest of the week in Christian religion and tradition is Sunday, many business and social calendars in the Western Christian world mark Sunday as the first day of the week.

ISO prescribes Monday as the first day of the week with ISO-8601.

Saturday and Sunday are commonly called the weekend and are days of rest and recreation in most western cultures. The other five days are then known as weekdays, a term which before Saturday got a similar secular work exemption applied to all days except the Sunday; compare Feria. Friday and Saturday are days of rest in some Muslim countries and Israel. The Jewish Sabbath lasts from Friday sunset to Saturday nightfall.

In some countries such as Iran, the weekend is only one day long (Friday) and the week starts on a Saturday. Other Muslim countries have weekends on Thursday and Friday.

The two-day weekend has become prevalent only during the twentieth century, leading to some calendars placing Sunday at the end of the week. The five-day working week, and some people mistaking Christian worship on Sunday for observance of the Sabbath day of rest has led most people in recent years to consider Monday to be the first day of the week.

Facts and figures

  • 1 week = 7 days = 168 hours = 10,080 minutes = 604,800 seconds (except at daylight saving time transitions or leap seconds)
  • 1 Gregorian calendar year = 52 weeks + 1 day (2 days in a leap year)
  • 1 week = 23.01% of an average month
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.

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

Week number

Weeks in a Gregorian calendar year can be numbered for each year. This style of numbering is commonly used (for example, by businesses) in some European and Asian countries, but rare elsewhere.

ISO 8601 includes the ISO week date system, a numbering system for weeks; each week is associated with the year in which Thursday occurs (so that if a year starts in a long weekend Friday–Sunday, week one of the year will start after that). Thus, for example, week 1 of 2004 (2004W01) ran from Monday 29 December 2003 to Sunday, 4 January 2004. The highest week number in a year may be 52 or 53.

The numbering system in different countries may deviate from the international ISO standard. There are at least six possibilities[2] [3]:

First day of week First week of year contains Weeks assigned twice Used by/in
Monday1 January,1st Sunday,1–7 days of yearyesUK
Monday4 January,1st Thursday,4–7 days of yearnoMost of Europe ISO 8601(1988), European Norm EN 28601 (1992)
Monday7 January,1st Monday,7 days of yearno
Wednesday1 January,1st Tuesday,1–7 days of yearyes
Saturday1 January,1st Friday,1–7 days of yearyes
Sunday1 January,1st Saturday,1–7 days of yearyesUSA

Liturgical week

In Christian liturgy, the week is mainly dominated by the special status of the Sunday.

The week was regarded as a sacred institution among the Jews owing to the law of the Sabbath rest and its association with the first chapter of Genesis. The earliest Christian converts seem tenacious of the usages (so far as they were compatible with the law of the Gospel) in which they had been brought up. The Sunday, "the first day of the week" (Acts 20:7; 1 Corinthians 16:2; cf. Revelation 1:10), soon replaced the Sabbath as the great day of religious observance, but the week itself remained as before. Indeed, there is much to recommend the idea that in the first and second centuries the only commemorations of the great Christian mysteries formed a weekly, not an annual, cycle. Sunday, according to the Epistle of Barnabas (xv), was "the beginning of another world", and the writer further says: "Wherefore also we keep the eighth day for rejoicing, in the which also Jesus rose from the dead and having been manifested ascended into the heavens". Again the Didache (viii) ordains: "Let not your fasts be with the hypocrites; for they fast on the second [Monday] and fifth [Thursday] days of the week, but do ye fast on the fourth [Wednesday] and on the day of preparation [Friday]", while in c. xiv we are told "And on the Day of the Lord come together and break bread and give thanks". Altogether it becomes clear from the language of Tertullian, the Apostolic Constitution and other early writers that the Sunday in each week was regarded as commemorating the Resurrection, and the Wednesday and Friday the betrayal and Passion of Christ.

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, "Hist. Eccl.", 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.

See also



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