Years are easy - there's the sun. And the moon determines months. Yes, both are approximations but they are close enough. The question is whether there is any actual physical basis for a week being 7 days. If it had to do with weather patterns, one would expect a different length of weeks in cultures in different climate zones. If you have a monsoonal pattern, for example, you might have a week that is shorter or longer than people have in temperate climates.
None of us knew the answer to this, but I wonder if there are (or have been) cultures in which a week is 6 days or 9 days or anything other than 7. Wikipedia is marginally helpful (and suggests the answer is "yes") but not entirely satisfying. This is more a subject for cocktail party chatter than actual serious inquiry, by the way.
By the way, I had sent an email to someone to cancel a meeting and titled it "need to cancel 21 Jan" without putting in the word "meeting". He replied that I shouldn't cancel January 21st because that would create the calendrical equivalent of a black hole. He did, however, give me permission to cancel April 15th.
