The origin of seven days per week appears to be astronomically related back to the Babylonians.
According to this ancient astronomy it is because there are 7 astronomical objects in the sky that can be observed. This includes:
- the sun
- the moon
- 5 planets (Jupiter, Venus, Saturn, Mercury and Mars)
It is also thought that 7 days is a simplification of a quarter of a lunar month.
A typical calendar week, starting on either Sunday or Monday.
Sources/references:
http://en.wikipedia.org/wiki/Week#Origin_of_the_seven-day_week