The story
Ancient societies divided time in many ways. Egypt used ten-day weeks, and the Etruscans and early Romans an eight-day market cycle. The Babylonians marked the 7th, 14th, 21st and 28th days of each lunar month as special days with restrictions, but their cycle restarted with every new moon. Ancient Israel kept something different: an unbroken cycle of seven days ending in the Sabbath, from the Hebrew word for rest or cessation, on which work stopped.
Scholars still debate how far Babylonian customs influenced the Sabbath, but many conclude that a week built around a regular day of rest, independent of the moon, was a distinctively Jewish contribution to civilisation. Jewish tradition itself regards the Sabbath as the people's first and most sacred institution, given special status in the opening chapters of Genesis.
The Ten Commandments give two reasons for it. In Exodus it recalls that God rested on the seventh day of creation. In Deuteronomy it is tied to freedom: "Remember that you were slaves in Egypt", therefore keep the Sabbath. Rest became a commandment rather than a privilege, a weekly reminder that people are not slaves to their work. To this day Shabbat begins at sunset on Friday with candles and blessings over wine and bread, and ends on Saturday night with the Havdalah ceremony; Jewish liturgy greets it as a bride and a queen.
As Jews settled across the Roman Empire, their customs became widely known, and Roman writers such as Seneca and Ovid mention the Jewish Sabbath. The early Christian seven-day week, centred on Sunday as the day of the Resurrection, grew out of the Jewish week centred on Saturday. It later merged with the Roman astrological week, which named the days after the Sun, the Moon and the five visible planets. In AD 321 the Emperor Constantine decreed a seven-day week across the Roman Empire and made Sunday a public holiday, and the pattern spread from there through Europe and then the rest of the world. Islam, too, keeps a weekly holy day, on Friday.
Today the seven-day week is the standard unit of time between the day and the month almost everywhere on earth, and the idea of a regular day off, now usually a weekend, is taken for granted by billions of people. Its roots lie in the ancient Israelite conviction that every person deserves a day of rest.
Why it matters
The idea of a regular, protected day of rest for everyone, rooted in the Sabbath, shaped the modern week and the weekend that billions of people enjoy.
Sources
Another act of tikkun olam. This is part of tikkun olam, the Jewish idea of repairing the world, and every one of us can add to it.