A lot of people on this thread are making deductions froma 28 day lunar cycle.Actually thelunar cycle is 29.53 days...30 to the nearest day.So 5 6-day, or 3 10-day, weeks in a month, for example, would be more accurate than 4 7-day weeks. However, 7 days is the closest whole-number approximation to a lunar quarter,which is a very obvious easy to observe unit of time (actually 7.38 days).