saw theory that the week is the yang cycle and the month is the yin cycle, i.e. masculine and feminine. free testosterone in males rises a bit for each day gone without ejaculation, peaks after about 7 days, then returns to baseline. this makes sense to me, both the month and the year are intuitive enough to grasp over time simply by looking at the sky but the week always seemed arbitrary, not so if it is related to a man's natural cycles just as the month is to a woman's.