Why does a week have seven days?

The Bible says that God created the Universe in six days and that the seventh was a day of rest. It was the ancient Jews who adopted the seven-day week and made one of them (the sabbath) a religious festival. The Romans dedicated Sunday to the Sun, Monday to the Moon, Tuesday to Mars, Wednesday to Mercury, Thursday to Jupiter, Friday to Venus, and Saturday to Saturn. The Christians later dedicated Sunday to the Lord (Domenicus).
In the English calendar, Monday came from the Old English for Moon day; Tuesday, Wednesday and Thursday were derived from the names of Germanic gods similar to the Roman ones. Friday is derived from a Norse and Teutonic goddess Frigg, or Frigga. Saturday comes from the Latin for Saturn.