Where did the seven-day week come from?

The week is not tied to any natural cycle the way the day, month and year are. Historians trace it to a blend of traditions: Babylonian reverence for the number seven, a continuous seven-day Sabbath cycle in Judaism, and a Roman planetary week named after the Sun, the Moon and five planets. Which of these came first, and how much each influenced the others, is debated. The Roman emperor Constantine made the seven-day week official in AD 321. [1] [2]

The earliest evidence of astrological significance for a seven-day period is a decree of King Sargon of Akkad around 2300 BC. The Akkadians venerated the number seven, and the key celestial bodies visible to the naked eye numbered seven: the Sun, the Moon and the five closest planets. [1] [3]

The Babylonians counted from the new moon and treated the 7th, 14th, 21st and 28th days of the lunar month as holy or inauspicious days, with at least the 28th a rest day. One popular account says they split a rounded 28-day cycle into four 7-day periods and used leap days to stay in step with the Moon. Wikipedia's account cautions that this was a lunar-month pattern, not a continuous week, and that the later practice of naming weekdays after planets did not come from the Babylonians. [4] [1]

A continuous seven-day cycle running independently of the Moon's phases was first practiced in Judaism, dated to the 6th century BC at the latest. Scholars disagree about where it came from. [1]

One view, associated with Friedrich Delitzsch, holds that the week was adopted from the Babylonians, who used intercalary days to align the last week of a month with the new moon, and that the Jews then removed the dependence on the Moon. Eviatar Zerubavel allows that the Sabbath may have been inspired by Babylonian customs during the exile and that the number seven may be Mesopotamian, but argues that a week built purely on the Sabbath is a distinctively Jewish innovation, since Mesopotamians themselves had no seven-day week. [1]

A third view, held by Niels-Erik Andreasen, Jeffrey Tigay and others, notes that the Sabbath appears in some of the earliest layers of the Pentateuch, dated to the 9th century BC at the latest, well before the exile. They find the resemblance to the Babylonian system weak and conclude that the Sabbath week may be an independent Israelite creation. No source in these results says which view most specialists hold. [1]

The Romans traditionally used an eight-day market cycle. After the Julian calendar took effect in 45 BC, a seven-day planetary week, with days tied to the Sun, the Moon and the five visible planets, came into increasing use. It has traditionally been seen as Hellenistic, but Ilaria Bultrighini and Sascha Stern argue it appeared earlier in Roman Italy and may have been loosely inspired by knowledge of the Jewish week. The origin of the planetary week is described as disputed. [1]

The seven-day week was widely known across the empire by the 1st century AD. The eight-day and seven-day weeks existed side by side until Constantine made the seven-day week official in AD 321, making the day of the Sun a public holiday. Christianity then merged the astrological week with the Jewish-derived week, and the system spread across Europe and eventually the world. [2] [1]