Why a Week Has Seven Days
Days come from the rotation of the Earth, months roughly from the orbit of the Moon, and years from the orbit of the Earth. The week matches nothing in the sky. It is a convention, and its history is visible in the words we use every day.
Seven visible objects
Ancient Babylonian astronomy tracked seven moving objects: the Sun, the Moon, and the five planets visible to the naked eye. Each hour was assigned to one of them in a repeating order, and the first hour of each day gave the day its name. The scheme travelled through the Hellenistic world and into Rome, which renamed the planets as gods.
The names you use now
Saturday, Sunday and Monday are transparently Saturn, the Sun and the Moon. Tuesday to Friday in English come from Germanic gods mapped onto Roman counterparts: Tiw, Woden, Thor and Frigg replace Mars, Mercury, Jupiter and Venus. In Romance languages the Roman names survive directly — mardi, mercredi, jeudi, vendredi.
Why it stuck
A seven-day cycle divides labour, rest and market days usefully, and it was adopted across very different societies. The seven-day week is now an international standard, along with the ISO rule that the week begins on Monday — a detail that still causes bugs in software.
Knowledge check
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Where do English weekday names Tuesday to Friday come from?
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