It is conjectured that the seven day week as we know it was developed by the
Babylonians over 3000 years ago.
Historians believe that the Babylonians were avid
astronomers. They based many of their
mathematical systems (including their calculations of time) on the movement of the heavenly bodies. The first convenient division of time to be devised (after the day) was the month because it could be tracked by observing the cycles of the
moon. It was at some point deemed convenient by Babylonian scholars to find a division of time greater than the day and shorter than the month. The big problem was that a cycle of the moon lasted about 29 and a half days. No round number of days divides evenly into 29 and a half - 4 was the best division they could find. So they went with the seven day week and worried about the loose change of extra days later.
Throughout the world various systems for the week had developed. The most popular competition for the seven day week was the 10 day week. Yet the ten day week proved to be too long for those looking forward to the weekend. Besides, adoption of the seven day week by the Hebrews would ensure its longevity over other forms; during the early Christian Era it grew in popularity with the popularity of both Judaism and Christianity. The Christians converted the Romans and the Romans forcibly converted the remainder of the civilized world. In this way the creation story, where God creates the universe in seven days and then rests, became a model for the activities of all mankind.
So today we still use the seven day week. Only now, as a tribute to man's productivity, we rest two days a week.