An excellent source you might consider to answer this question is Daniel Boorstin's The Discoverers, A History of Man's Search to Know his World and Himself." Prof Boorstin offers the following comment:
"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 eight day--the market day. This was a day of rest and festivity, a school holiday....The number seven almost everywhere has special charm. The Japanese found seven gods of happiness. Rome was set on seven hills, the ancients counted seven wonders of the world and medieval Christians enumerated seven deadly sins. ...By the early third century A.D., Romans were living with a seven day week."
Professor Boorstin also indicates that origins of the seven day week are unclear. The Book of Genesis in the Old Testament seems to indicate a seven day week by designating the seventh day as a day of rest; and that the world was created in seven days. The Romans believed there were seven planets, and each day was governed by a particular planet. The Sun and Moon were two of those seven planets. The days of the week came from the name of the controlling planet.. Sunday, "Moon-Day," Tuesday controlled by Mars (Mardi in French) Wednesday by Mercury (Mercridi) Friday by Venus (Vendredi) and Saturns-day.