A week is a time unit equal to seven days. It is the standard time period used for cycles of rest days in most parts of the world, mostly alongside—although not strictly part of—the Gregorian calendar.
In many languages, the days of the week are named after classical planets or gods of a pantheon. In English, the names are Sunday, Monday, Tuesday, Wednesday, Thursday, Friday, and Saturday. Such a week may be called a planetary week. This arrangement is similar to a week in the Bible in which the seven days are simply numbered with the first day being a Christian day of worship and the seventh day being a sabbath day. The traditional Biblical sabbath is aligned with Saturday.
While, for example, the United States, Canada, Brazil, Japan and other countries consider Sunday as the first day of the week, and while the week begins with Saturday in much of the Middle East, the international ISO 8601 standard has Monday as the first day of the week. The ISO standard includes the ISO week date system, a numbering system for weeks within a given year, where each week starting on a Monday is associated with the year that contains that week's Thursday. 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, the "work week", or "school week" referring only to the days spent on those activities.


The English word :wikt:week|week comes from the Old English wice, ultimately from a Common Germanic wikōn-, from a root wik- "turn, move, change". The Germanic word probably had a wider meaning prior to the adoption of the Roman calendar, perhaps "succession series", as suggested by Gothic wikō translating taxis "order" in Luke 1:8.
The seven-day week is named in many languages by a word derived from "seven". The archaism sennight preserves the old Germanic practice of reckoning time by nights, as in the more common fortnight. Hebdomad and hebdomadal week both derive from the Greek hebdomás. Septimana is cognate with the Romance terms derived from Latin .
Slavic has a formation :wikt:Reconstruction:Proto-Slavic/tъdьnь|*tъdьnь, from *tъ "this" + *dьnь "day". Chinese has :wikt:星期|星期, as it were ":wikt:星|planetary :wikt:期|time unit".

Definition and duration

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 or 52.1775 weeks. There are exactly 20,871 weeks in 400 Gregorian years, so was a just as was .
Relative to the path of the Moon, a week is 23.659% of an average lunation or 94.637% of an average quarter lunation.
Historically, the system of dominical letters has been used to facilitate calculation of the day of week.
The day of the week can be easily calculated given a date's Julian day number :
Adding one to the remainder after dividing the Julian day number by seven yields that date's ISO 8601 day of the week. For example, the Julian day number of is. Calculating yields, corresponding to.

Days of the week

The days of the week were originally named for the classical planets. This naming system persisted alongside an "ecclesiastical" tradition of numbering the days, in ecclesiastical Latin beginning with dominica as the first day. The Greco-Roman gods associated with the classical planets were rendered in their interpretatio germanica at some point during the late Roman Empire, yielding the Germanic tradition of names based on indigenous deities.
The ordering of the weekday names is not the classical order of the planets. Instead, the planetary hours systems resulted in succeeding days being named for planets that are three places apart in their traditional listing. This characteristic was apparently discussed in Plutarch in a treatise written in c. AD 100, which is reported to have addressed the question of Why are the days named after the planets reckoned in a different order from the actual order?.
Greco-Roman deityHelios-SolSelene-LunaAres-MarsHermes-MercuryZeus-JupiterAphrodite-VenusCronus-Saturn
Greek:ἡμέρα Ἡλίουἡμέρα Σελήνηςἡμέρα Ἄρεωςἡμέρα Ἑρμοῦἡμέρα Διόςἡμέρα Ἀφροδίτηςἡμέρα Κρόνου
Latin::la:Dies Solis|dies Sōlis:la:Dies Lunae|dies Lūnae:la:Dies Martis|dies Martis:la:Dies Mercurii|dies Mercuriī:la:Dies Iovis|dies Iovis:la:Dies Veneris|dies Veneris:la:Dies Saturni|dies Saturnī
interpretatio germanicaSunMoonTiwazWodanazÞunrazFrige
Old Englishsunnandægmōnandægtiwesdægwōdnesdægþunresdæg frīgedægsæterndæg

An ecclesiastical, non-astrological, system of numbering the days of the week was adopted in Late Antiquity. This model also seems to have influenced the designation of Wednesday as "mid-week" in Old High German and Old Church Slavonic. Old Church Slavonic may have also modeled the name of Monday, понєдѣльникъ, after the Latin feria secunda. The ecclesiastical system became prevalent in Eastern Christianity, but in the Latin West it remains extant only in modern Icelandic, Galician, and Portuguese.
1. Sunday 2. Monday3. Tuesday4. Wednesday5. Thursday6. Friday 7. Saturday
GreekΚυριακὴ ἡμέρα
/kiriaki iméra/
Δευτέρα ἡμέρα
/devtéra iméra/
Τρίτη ἡμέρα
/tríti iméra/
Τετάρτη ἡμέρα
/tetárti iméra/
Πέμπτη ἡμέρα
/pémpti iméra/
Παρασκευὴ ἡμέρα
/paraskevi iméra/
Latin dominica;
rarely feria prima, feria dominica
feria secundaferia tertiaferia quarta;
rarely media septimana
feria quintaferia sextaSabbatum; dies sabbatinus, dies Sabbati;
rarely feria septima, feria Sabbati


A continuous seven-day cycle that runs throughout history without reference to the phases of the moon was first practised in Judaism, dated to the 6th century BC at the latest.
There are several hypotheses concerning the origin of the biblical seven-day cycle.
Friedrich Delitzsch and others suggested that the seven-day week being approximately a quarter of a lunation is the implicit astronomical origin of the seven-day week, and indeed the Babylonian calendar used intercalary days to synchronize the last week of a month with the new moon. According to this theory, the Jewish week was adopted from the Babylonians while removing the moon-dependency.
However, Niels-Erik Andreasen, Jeffrey H. Tigay, and others claimed that the Biblical Sabbath is mentioned as a day of rest in some of the earliest layers of the Pentateuch dated to the 9th century BC at the latest, centuries before Judea's Babylonian exile. They also find the resemblance between the Biblical Sabbath and the Babylonian system to be weak. Therefore, they suggested that the seven-day week may reflect an independent Israelite tradition. Tigay writes:
It is clear that among neighboring nations that were in position to have an influence over Israel – and in fact which did influence it in various matters – there is no precise parallel to the Israelite Sabbatical week. This leads to the conclusion that the Sabbatical week, which is as unique to Israel as the Sabbath from which it flows, is an independent Israelite creation.

The seven-day week seems to have been adopted, at different stages, by the Persian Empire, in Hellenistic astrology, and in Gupta India and Tang China.
The Babylonian system was received by the Greeks in the 4th century BC. However the designation of the seven days of the week to the seven planets is an innovation introduced in the time of Augustus. The astrological concept of planetary hours is rather an original innovation of Hellenistic astrology, probably first conceived in the 2nd century BC.
The seven-day week was widely known throughout the Roman Empire by the 1st century AD, along with references to the Jewish Sabbath by Roman authors such as Seneca and Ovid. When the seven-day week came into use in Rome during the early imperial period, it did not immediately replace the older eight-day nundinal system. The nundinal system had probably fallen out of use by the time Emperor Constantine adopted the seven-day week for official use in CE 321, making the Day of the Sun a legal holiday.

Ancient Near East

The earliest evidence of an astrological significance of a seven-day period is connected to Gudea, priest-king of Lagash in Sumer during the Gutian dynasty, who built a seven-room temple, which he dedicated with a seven-day festival. In the flood story of the Assyro-Babylonian epic of Gilgamesh, the storm lasts for seven days, the dove is sent out after seven days, and the Noah-like character of Utnapishtim leaves the ark seven days after it reaches firm ground.
It is possible that the Hebrew seven-day week is based on the Babylonian tradition, although going through certain adaptations. George Aaron Barton speculated that the seven-day creation account of Genesis is connected to the Babylonian creation epic, Enûma Eliš, which is recorded on seven tablets.
Counting from the new moon, the Babylonians celebrated the 7th, 14th, 21st and 28th as "holy-days", also called "evil days". On these days, officials were prohibited from various activities and common men were forbidden to "make a wish", and at least the 28th was known as a "rest-day".
On each of them, offerings were made to a different god and goddess.
In a frequently-quoted suggestion going back to the early 20th century, the Hebrew Sabbath is compared to the Sumerian sa-bat "mid-rest", a term for the full moon. The Sumerian term has been reconstructed as rendered Sapattum or Sabattum in Babylonian, possibly present in the lost fifth tablet of the Enûma Eliš, tentatively reconstructed "bbath shalt thou then encounter, midly".

Achaemenid period

The Zoroastrian calendar follows the Babylonian in relating the 7th, 14th, 21st, and 28th of the month to Ahura Mazda.
The forerunner of all modern Zoroastrian calendars is the system used to determine dates in the Persian Empire, adopted from the Babylonian calendar by the 4th century BC.
Frank C. Senn in his book Christian Liturgy: Catholic and Evangelical points to data suggesting evidence of an early continuous use of a seven-day week; referring to the Jews during the Babylonian captivity in the 6th century BC, after the destruction of the Temple of Solomon.
While the seven-day week in Judaism is tied to Creation account in the Book of Genesis in the Hebrew Bible, it is not clear whether the Genesis narrative predates the Babylonian captivity of the Jews in the 6th century BC. At least since the Second Temple period under Persian rule, Judaism relied on the seven-day cycle of recurring Sabbaths.
Tablets from the Achaemenid period indicate that the lunation of 29 or 30 days basically contained three seven-day weeks, and a final week of eight or nine days inclusive, breaking the continuous seven-day cycle.
The Babylonians additionally celebrated the 19th as a special "evil day", the "day of anger", because it was roughly the 49th day of the month, completing a "week of weeks", also with sacrifices and prohibitions.
Difficulties with Friedrich Delitzsch's origin theory connecting Hebrew Shabbat with the Babylonian lunar cycle include reconciling the differences between an unbroken week and a lunar week, and explaining the absence of texts naming the lunar week as Shabbat in any language.

Hellenistic and Roman era

In Jewish sources by the time of the Septuagint, the term "Sabbath" by synecdoche also came to refer to an entire seven-day week, the interval between two weekly Sabbaths.
Jesus's parable of the Pharisee and the Publican describes the Pharisee as fasting "twice in the week".
The ancient Romans traditionally used the eight-day nundinum but, after the Julian calendar had come into effect in 45 BC, the seven-day week came into increasing use. For a while, the week and the nundinal cycle coexisted, but by the time the week was officially adopted by Constantine in AD 321, the nundinal cycle had fallen out of use. The association of the days of the week with the Sun, the Moon and the five planets visible to the naked eye dates to the Roman era.
The continuous seven-day cycle of the days of the week can be traced back to the reign of Augustus; the first identifiable date cited complete with day of the week is 6 February AD 60, identified as a "Sunday" in a Pompeiian graffito. According to the Julian calendar, 6 February 60 was, however, a Wednesday. This is explained by the existence of two conventions of naming days of the weeks based on the planetary hours system: 6 February was a "Sunday" based on the sunset naming convention, and a "Wednesday" based on the sunrise naming convention.

Adoption in Asia

China and Japan

The earliest known reference in Chinese writings to a seven-day week is attributed to Fan Ning, who lived in the late 4th century in the Jin Dynasty, while diffusions from the Manichaeans 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 variant of the planetary system was brought to Japan by the Japanese monk Kūkai. 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 Period.


The seven-day week was known in India by the 6th century, referenced in the Pañcasiddhāntikā. Shashi mentions the Garga Samhita, which he places in the 1st century BC or AD, as a possible earlier reference to a seven-day week in India. He concludes "the above references furnish a terminus ad quem The terminus a quo cannot be stated with certainty".


In Arabia, a similar seven-week system was adopted, that may be influenced by the Hebrew week.

Christian Europe

The seven-day weekly cycle has remained unbroken in Christendom, and hence in Western history, for almost two millennia, despite changes to the Coptic, Julian, and Gregorian calendars, demonstrated by the date of Easter Sunday having been traced back through numerous computistic tables to an Ethiopic copy of an early Alexandrian table beginning with the Easter of AD 311.
A tradition of divinations arranged for the days of the week on which certain feast days occur develops in the Early Medieval period. There are many later variants of this, including the German Bauern-Praktik and the versions of Erra Pater published in 16th to 17th century England, mocked in Samuel Butler's Hudibras. South and East Slavic versions are known as koliadniki, with Bulgarian copies dating from the 13th century, and Serbian versions from the 14th century.
Medieval Christian traditions associated with the lucky or unlucky nature of certain days of the week survived into the modern period. This concerns primarily Friday, associated with the crucifixion of Jesus. Sunday, sometimes personified as Saint Anastasia, was itself an object of worship in Russia, a practice denounced in a sermon extant in copies going back to the 14th century.
Sunday, in the ecclesiastical numbering system also counted as the feria prima or the first day of the week; yet, at the same time, figures as the "eighth day", and has occasionally been so called in Christian liturgy.
Justin Martyr wrote: "the first day after the Sabbath, remaining the first of all the days, is called, however, the eighth, according to the number of all the days of the cycle, and remains the first."
A period of eight days, usually starting and ending on a Sunday, is called an octave, particularly in Roman Catholic liturgy. In German, the phrase heute in acht Tagen means one week from today. The same is true of the Italian phrase oggi otto.

Week numbering

Weeks in a Gregorian calendar year can be numbered for each year. This style of numbering is often used in European and Asian countries. It is less common in the U.S. and elsewhere.

The ISO week date system

The system for numbering weeks is the ISO week date system, which is included in ISO 8601. This system dictates that each week begins on a Monday and is associated with the year that contains that week's Thursday.

Determining ''Week 1''

In practice week 1 of any year can be determined as follows:
It is also possible to determine if the last week of the previous year was Week 52 or Week 53 as follows:
In some countries, though, the numbering system is different from the ISO standard. At least six numberings are in use:
The semiconductor package date code is often a 4 digit date code YYWW where the first two digits YY are the last 2 digits of the calendar year and the last two digits WW are the two-digit week number.
The tire date code mandated by the US DOT is a 4 digit date code WWYY with two digits of the week number WW followed by the last two digits of the calendar year YY.

"Weeks" in other calendars

The term "week" is sometimes expanded to refer to other time units comprising a few days. Such "weeks" of between four and ten days have been used historically in various places. Intervals longer than 10 days are not usually termed "weeks" as they are closer in length to the fortnight or the month than to the seven-day week.

Pre-modern calendars

Calendars unrelated to the Chaldean, Hellenistic, Christian, or Jewish traditions often have time cycles between the day and the month of varying lengths, sometimes also called "weeks".
An eight-day week was used in Ancient Rome and possibly in the pre-Christian Celtic calendar. Traces of a nine-day week are found in Baltic languages and in Welsh. The ancient Chinese calendar had a ten-day week, as did the ancient Egyptian calendar.
A six-day week is found in the Akan Calendar. Several cultures used a five-day week, including the 10th century Icelandic calendar, the Javanese calendar, and the traditional cycle of market days in Korea. The Igbo have a "market week" of four days. Evidence of a "three-day week" has been derived from the names of the days of the week in Guipuscoan Basque.
The Aztecs and Mayas used the Mesoamerican calendars. The most important of these calendars divided a ritual cycle of 260 days into 20 weeks of 13 days. They also divided the solar year into 18 periods of 20 days and five nameless days, creating a 20-day month divided into four five-day weeks. The end of each five-day week was a market day.
The Balinese Pawukon is a 210-day calendar consisting of 10 different simultaneously running weeks of 1, 2, 3, 4, 5, 6, 7, 8, 9, and 10 days, of which the weeks of 4, 8, and 9 days are interrupted to fit into the 210-day cycle.

Modern calendar reforms

A 10-day week, called décade, was used in France for nine and a half years from October 1793 to April 1802; furthermore, the Paris Commune adopted the Revolutionary Calendar for 18 days in 1871.
The Bahá'í calendar features a 19-day period which some classify as a month and others classify as a week.
The International Fixed Calendar fixed every date always on the same weekday. This plan kept a 7-day week while defining a year of 13 months with 28 days each. It was the official calendar of the Eastman Kodak Company for decades.
In the Soviet Union between 1929 and 1940, most factory and enterprise workers, but not collective farm workers, used five- and six-day work weeks while the country as a whole continued to use the traditional seven-day week. From 1929 to 1951, five national holidays were days of rest. From autumn 1929 to summer 1931, the remaining 360 days of the year were subdivided into 72 five-day work weeks beginning on. Workers were assigned any one of the five days as their day off, even if their spouse or friends might be assigned a different day off. Peak usage of the five-day work week occurred on at 72% of industrial workers. From summer 1931 until, each Gregorian month was subdivided into five six-day work weeks, more-or-less, beginning with the first day of each month. The sixth day of each six-day work week was a uniform day of rest. On 74.2% of industrial workers were on non-continuous schedules, mostly six-day work weeks, while 25.8% were still on continuous schedules, mostly five-day work weeks. The Gregorian calendar with its irregular month lengths and the traditional seven-day week were used in the Soviet Union during its entire existence, including 1929–1940; for example, in the masthead of Pravda, the official Communist newspaper, and in both Soviet calendars displayed here. The traditional names of the seven-day week continued to be used, including "Resurrection" for Sunday and "Sabbath" for Saturday, despite the government's official atheism.