Eight Days a Week
The following is from Daniel J. Boorstin's book The Discoverers: A History of Man's Search to Know His World and Himself.

Our own Western 7-day week, one of the most arbitrary of our institutions, came into being from popular and spontaneous agreement, not from a law or the order of any government. How did it happen? Why? When?

Why a seven-day week?

The ancient Greeks, it seems, had no week. Romans lived by an 8-day week. Farmers who worked in the fields for 7 days came to town for the eighth day -- the market day (or nundinae). This was a day of rest and festivity, a school holiday, the occasion for public announcements and for entertaining friends. When and why the Romans fixed on 8 days and why they eventually changed to a 7-day week is not clear. The number seven almost everywhere has had a special charm. The Japanese found seven gods of happiness, Rome was set on seven hills, the ancients counted seven deadly sins. The Roman change from eight to seven seems not to have been accomplished by any official act. By the early third century A.D. Romans were living with a 7-day week.