The Origin of the Mystical Number Seven in Mesopotamian Culture;   Division by Seven in the Sexagesimal Number System
  In the middle of the third millennium BC the Sumerians must have noticed that the reciprocal of the number 7,in contrast to the numbers 1,2,3,4,5,and 6,could not be expressed by a finite sexagesimal fraction but it recurred every three places.Since the number 7 is the first natural number that has such a property, it was of particular interest to them and became the representative of their number mysticism. 
Kazuo Muroi
math.HO