Why are there seven days in a week?

The seven-day week has no astronomical basis whatsoever. The moon takes about 29.5 days to go through its phases and the year takes about 365. Neither number divides by seven. The week is purely an agreement people made.

It started in ancient Babylon, about 3,300 years ago. With the naked eye, people could pick out seven lights that moved against the fixed stars: the Sun, the Moon, and the five planets Mars, Mercury, Jupiter, Venus and Saturn. These were treated as seven bodies taking turns on duty, and seven days became one full round.

Two forces carried it out into the world.

1. The Jewish Sabbath. Six days of work followed by a seventh day of rest gave the week its rhythm of effort and recovery.
2. The Roman Empire. On 7 March, AD 321, the emperor Constantine made the day of the Sun the first day of the week by law. That made the seven-day week official, and it travelled with Christianity and with empire to the rest of the world.

The Chinese word for week, 星期, is a direct translation of the "seven lights". In Japanese, the days are still named Sun, Moon, Fire, Water, Wood, Metal, Earth — the same seven.

In one line: the week is an invention — first seven lights were counted, then a religious tradition was layered on, then a single law nailed it down.

A stranger fact: China only adopted the seven-day week around the beginning of the 20th century, as Western learning spread. Before that, time was divided by the ten-day period called 旬.