Days, months, and years were given to us by nature, but we invented the week for ourselves. There is nothing inevitable about a seven-day cycle, or about any other kind of week, it represents an arbitrary rhythm imposed on our activities, unrelated to anything in the natural order. But where the week exists--and there have been many cultures where it doesn't--it is so deeply embedded in our experience that we hardly ever question its rightness, or think of it as an artificial convention, for most of us it is a matter of 'second nature.