There's a good answer here from /u/yodatsracist about the origins of weeks. From his comment:

The gap between market days hasn't been constant throughout time and space, however. We're so used to the seven day cycle that it seems natural, but there's no natural reason for seven days rather than six or eight or any other number around there.

Zerubavel traces the origin of our particular seven-day week back to two reinforcing sources: one, the seven-day Jewish week, which obviously is only a footnote until it the rise Christianity and Islam, who only slightly adapted the Jewish week. Christians moved their holy day to Sunday, Muslims moved their holy day to Friday. Once the holy day is tied to a seven p-day cycle, there's really no changing it. You can't just say "We're moving God's day two days back this week." And once you're on a seven-day religious cycle, it's hard to move to an economic cycle of any other length, because the religious day (often a day with limited work) will end up interfering with the economic cycle.