A lunar month (28 days) is a slightly useful and easily measured unit of time, but a bit too long for a lot of purposes. But it only splits into 47 and 214, so a shorter measure would be 2 days (not so useful), 4 days (maybe), 7 days (maybe) and 14 days (too long). A seven day measure aligns nicely with the moon's quarters as well. That's what I had always assumed was the reason for a 7 day week; I wonder if the celestial bodies reasoning is more of a religious cover story for something people were already doing (this is pure conjecture on my part)