Why are there 7 days in a week?
The seven-day week comes from ancient Babylon, where astronomers linked it to the seven visible celestial bodies and the phases of the Moon. It spread through Jewish, Christian and Roman traditions.
The Babylonians observed seven moving objects in the sky: the Sun, the Moon, Mercury, Venus, Mars, Jupiter and Saturn. They also noticed that each phase of the Moon lasts roughly seven days. The Jewish tradition of a seven-day week with a day of rest, the Sabbath, also played a central role, and it later shaped Christian and Islamic calendars.
The Romans adopted the seven-day week, naming days after the seven celestial bodies. In 321 AD, Emperor Constantine made Sunday an official day of rest in the Roman Empire. The seven-day cycle has continued without interruption for centuries.
Day names
In English, Sunday, Monday and Saturday are named after the Sun, Moon and Saturn. Tuesday, Wednesday, Thursday and Friday come from Norse and Germanic gods: Tiw, Woden, Thor and Frigg. In Spanish and French, many day names still come from Roman gods and planets, like martes for Mars.
Did You Know?
The Soviet Union experimented with five-day and six-day weeks between 1929 and 1940 before returning to the seven-day week.