Except that I've read that the pre-Christian Romans had an 8 day week. Nor is there really any obvious association between the number of heavenly bodies and the days of the week since they aren't necessarily visible at the same time at a given latitude.

It seems that anthropologists define weeks by when market days are held. Which means they end up with anywhere from 3 to 8 days for different cultures. But that isn't very satisfying as it doesn't answer why market days are as far apart as they are.