In Christendom, as in Judaism and Islam, the seven-day cycle of days of the week marks the basic period of work, rest and communal worship. In the Bible, the Creation of the world takes six days, and God rests on the seventh.
It is difficult to trace the ultimate origin of the seven-day week, but in the Babylonian calendar, the 7th, 14th, 19th, 21st and 28th days of each month were set aside for rest. After the Exile, the Jewish calendar adopted the names of the Babylonian months, and it is possible that the week was also introduced into Judaism at this time.
In early Rome, there was an eight-day cycle between market days. It was only in the second century B.C. that a seven-day cycle became predominant, and this may have owed more to astrology than to Hebrew or Babylonian influences. Astrologers recognised seven planets (including the Sun and Moon) and assigned one planet to rule each of the 24 hours of the day, in a continuous sequence. The planet which ruled the first hour of the day was taken to rule the whole day, and this gave rise to a seven-day cycle.
The Romans began to name each day after its ruling planet: Saturn's day, the Sun's day, the Moon's day, Mars's day, Mercury's day, Jupiter's day, Venus's day. In the Romance languages, the connection is still evident. In French, for example, Monday to Friday become lundi, mardi, mercredi, jeudi, vendredi. In the Germanic languages, the names of the Norse gods Tiu, Woden, Thor and Freya replaced Mars, Mercury, Jupiter and Venus.
Jewish tradition originally had no names for the days of the week, giving them numbers instead. Only the Sabbath had its own name. The Roman names were adopted only slowly and reluctantly by the Jews and early Christians.
It is impossible to say whether the cycle of days of the week has continued without interruption since Roman times. The Gregorian calendar reform, though it removed ten days from the calendar at a stroke, nevertheless maintained the sequence of days of the week.