Why Are There Seven Days in a Week?

Why doesn't the week follow a natural cycle?

The day, the month and the year each follow something in the sky: the day comes from Earth turning on its axis, the month from the Moon circling Earth, and the year from Earth orbiting the Sun.^[3] The week has no such basis: E. G. Richards, in a chapter on calendars published by the U.S. Naval Observatory, lists it among cycles with "no astronomical basis."^[3] Seven days do not divide evenly into a lunar month or a 365-day year.^[2][3]

In the ancient world, weeks lasted anywhere from five to sixteen days.^[2] The Romans had an eight-day market cycle, and calendars in Central America used cycles of 13 and 20 days.^[2][3] Even so, the seven-day week has been used for millennia from China to Europe.^[1] Most of the world uses it today, and it has religious significance for Christians, Jews and Muslims.^[3]

Where did the seven-day week come from?

Sources place the origin of the seven-day week in the ancient Middle East but tell the story differently.^[1][2][3]

• A Babylonian origin. Royal Museums Greenwich traces the week to the Babylonians and a decree of King Sargon I of Akkad around 2300 BC, noting that they revered the number seven.^[1] Richards says the week probably began in Babylon as a useful but inexact division of the lunar month.^[3]
• The Jewish week. Richards says the Jewish people probably adopted the week during the Babylonian exile in the sixth century BC.^[3] Craig Harline, in his history of Sunday, writes that the Jewish week and its Sabbath may date back to around 1000 BC and were certainly in place around 587 BC.^[2] Except for the Sabbath and the day before it, Jewish weekdays were numbered, not named.^[2]
• The planetary week. Harline writes that Babylonian observers were tracking seven "planets" by 600 BC, but that the Babylonians themselves preferred lunar months.^[2] In his account, the week of seven days tied to the planets came from the later Greek, or Hellenistic, world in the second century BC.^[2] The American Heritage Dictionary likewise says this astrological system reached Hellenistic Egypt from Mesopotamia.^[9]

Which came first, the Jewish week or the planetary week, is not settled.^[2] Some scholars think the Jewish week influenced the Babylonians, while more think the Jews took the number seven from Babylon, and Harline concludes that the surviving evidence is unlikely ever to settle it.^[2]

Why seven? The seven "planets"

Before telescopes, the key bodies in the sky numbered seven: the Sun, the Moon and the five planets visible to the naked eye.^[1] In Babylonia they were tracked largely for astrology, because each was thought to be ruled by a god or goddess who influenced events on Earth.^[2]

Hellenistic observers set the number of days at seven to match the planets and placed each day under one of them.^[2] They also divided the day into 24 hours, each ruled by a planet in turn.^[2][3] According to an ancient account cited by Richards, the planet assigned to the first hour of a day gave that day its name, producing the order of days still used today.^[3][9]

How did Rome adopt the week?

The Roman calendar long had no weekly holy day.^[2] Sources disagree on when Rome took up the seven-day week: Richards says the cycle was widely used in the Roman Empire from the first century BC, while Harline dates Rome's own week, shaped by Jewish, planetary and Roman traditions, to the first century AD.^[3][2] The American Heritage Dictionary instead says the ancient Romans did not divide their calendar into weeks and credits Constantine the Great with grafting the astrological system onto it in AD 321.^[9]

In the planetary week, Saturn's day came first.^[2] By at least AD 100, Romans counted it as the seventh, which made the Sun's day the first.^[2] By at least AD 150, most Christians were gathering to worship on that first day, the Lord's Day.^[2] In AD 321, Constantine issued an edict against work on "the venerable day of the Sun," with an exception for farm labor.^[5]

Where do the English day names come from?

The names began with the Romans, who used the Latin words for the Sun, the Moon and the five known planets.^[4] In Old English, four of the Roman gods were swapped for Germanic gods seen as their counterparts, such as Woden for Mercury.^[9]

The English form of Thursday was influenced by the Old Norse name Thōrsdagr.^[9]

Has anyone tried to change the seven-day week?

In revolutionary France, a new calendar adopted in 1793 and counted from the founding of the Republic in 1792 split every month into three weeks of ten days.^[3][6] Days were numbered from primidi to décadi, replacing names such as Dimanche and Samedi, and people waited nine days for a weekend instead of six.^[6] The sociologist Eviatar Zerubavel argues that the calendar was meant to disrupt churchgoing and that its makers underestimated how deeply religious feeling was rooted.^[6] Peasants proved harder to win over than Parisians.^[6] Napoleon abolished the calendar on January 1, 1806.^[6][3]

In the Soviet Union, a decree of September 24, 1929, set a five-day working week of four workdays and one rest day.^[8] Workers were split into groups with different rest days, so family members and friends could not count on resting together.^[7] After problems such as overused machines breaking down and supply bottlenecks, the government switched in 1931 to a six-day week with a shared rest day on the sixth, twelfth, eighteenth, twenty-fourth and thirtieth of each month, and that system lasted until June 26, 1940, when the country returned to the seven-day week.^[7]

Scholars disagree about the Soviet reform's purpose: Zerubavel sees its attack on religion as a key part of its appeal to the Soviet leadership, while Tony Wood, writing in Cabinet magazine, argues it was more an attempt to fit daily life to the demands of industry.^[7] Zerubavel also notes that much of the countryside ignored the new calendars and kept to the seven-day rhythm.^[7]