If we go deep into the history of seven days we will find that there is no definite theory of why there are seven days in a week. Everyone has different opinions about it.
Some people say that week of seven days was first practiced by roman empire and then by Christians churches. After this British took the calendar as it was used by Romans and Christians. Slowly it spreads all over world and now every country is using the same theory that a week has seven days in which they manage their different tasks and schedules.
Others believe in religious theories and found its reason in the Bible. As It explains that god has created world in just six days and after his creation he took rest on the seventh day. So Jewish started taking this seventh day as the day of rest.
Another practical and geometrical theory about it is that if we took seven circular objects and wrap them with a rubber band we will observe an exact hexagon by placing the seventh one in the middle. It will be a stable geometrical figure while if we wrap three or more circular objects they will easily change from one geometrical figure to another. In ancient times logs of firewood, poles and other circular objects might be tied like. That can be one reason why this number got so much of significance.
One more common theory about it is that in ancient times people were aware of only seven planets: – sun, moon, mars, mercury, Jupiter, Venus and Saturn and took these planets as gods because in early times people only considered moving things as living but they did not know why these planets move and hence took them as supernatural powers. So they might have taken one day for each god and resulted in a week of seven days. In spite of all these theories we have no strong, definite and logical reason that why a week consists of seven days.