Why? What things need to happen every exact 7 days? Isn't it rather the case that we do some things weekly because that is a conventional unit of time? And if

Why? What things need to happen every exact 7 days? Isn't it rather the case that we do some things weekly because that is a conventional unit of time? And if you are designing something so that some process needs to be performed on it every week, you should be building some tolerance into that time-period. Which the eight day span, once every year, would test.