The Economist is running an interesting article on the history of the seven day cycle. Predictably, we owe our structural base to Ur:

The year, the day and (not quite so obviously) the month are natural divisions of time. The week is an oddity. The moon’s four phases are a near miss, but still a misfit, for weeks. You will be in trouble (like H.G. Wells’s ‘The Man Who Could Work Miracles’) if you try to make the moon perform every 28 days, instead of its usual 29