Because of the moons phases i think

its date back to the bible....(first page)

Digging into the history of the 7-day week is a very complicated matter. Authorities have very different opinions about the history of the week, and they frequently present their speculations as if they were indisputable facts. The only thing we seem to know for certain about the origin of the 7-day week is that we know nothing for certain. The common explanation is that the seven-day week was established as imperial calendar in the late Roman empire and furthered by the Christian church for historical reasons. The British Empire used the seven-day week and spread it worldwide. Today the seven-day week is enforced by global business and media schedules, especially television and banking. The first pages of the Bible explain how God created the world in six days and rested on the seventh. This seventh day became the Jewish day of rest, the sabbath, Saturday. Extra-biblical locations sometimes mentioned as the birthplace of the 7-day week include: Babylon, Persia, and several others. The week was known in Rome before the advent of Christianity. There are practical geometrical theories as well. For example, if you wrap a rubber band around 7 soda cans (or any other convenient circular objects). You get a perfect hexagon with the 7th can in the middle. It is the only stable configuration of wrapping more than 3 circular objects. Four, 5, and 6 objects will slip from one configuration to another. Ancients wrapping tent poles, small logs for firewood, or other ciruclar objects might have come upon this number and attach a mystical significance to it. One viable theory correlates the seven day week to the seven (astrological) "planets" known to the ancients: Sun, Moon, Mars, Mercury, Jupiter, Venus, and Saturn. The number seven does not seem an obvious choice to match lunar or solar periods, however. A solar year could be more evenly divided into weeks of 5 days, and the moon phases five-day and six-day weeks make a better short term fit (6 times 5 is 30) to the lunar (synodic) month (of about 29.53 days) than the current week (4 times 7 is 28). The seven-day week may have been chosen because its length approximates one moon phase (one quarter = 29.53 / 4 = 7.3825). http://webexhibits.org/calendars/week.html

I strongly believe to the point "One viable theory correlates the seven day week to the seven (astrological) "planets" known to the ancients: Sun, Moon, Mars, Mercury, Jupiter, Venus, and Saturn." Because, Indian astrology which dates back more than double the years of Bible, uses this method, and every day is associated with the astrological planets. I am not trying to proove any credibility for astrology, but such a system was prevailing 5000years back in India.

And on the seventh day God rested.

There are seven...O' no.