I have always just assumed that the natural celestial cycles (perhaps lunar) determined the 7 day week.  Imagine my surprise to learn that God set this number arbitrarily and then gave it to man!

A close friend of mine made me curious about this very question by claiming that the 7 day week was support for the evangelical protestant theological position, so I decided to look it up.  I thought surely God would have a better reason (like cosmological balance, or some other feature of nature).  Well, after a bit of searching, it seems that there is not very much agreement about the origin of the 7 day week.  The disagreements mostly revolve around which civilization had it first, but there is also pretty clear evidence it may have also just popped up simultaneously in several places.  Note here, that there have been and continue to be many different formulations of “the week”.   Not only is it possibly quite arbitrary indeed, but it seems that God gave this arbitrary number to numerous civilizations via their religion, rather than nature alone.

First, realize that we do NOT use the same 7 day week that the ancient Israelites did (they used some kind of ‘isolunar system’, whatever that is).  Second, most scholars agree that what they developed derived from earlier civilizations such as the Persians, Babylonians, or Sumerians (there is even one theory that it came from Egyptians).  The point is that culture almost never originates entirely from scratch. It comes from PEOPLE who came from other people, who came from people before them, moving around at times, migrating, or staying in one place for a while, but always coming from other people who also had culture.  If you find some very solid, very well preserved cultural ‘thing’ in a group of people, there’s a strong chance it derived in large part from various other peoples who came before them.  As you go further back to the most primitive original specimens of “modern human”, there’s a lot less of that culture, and fewer cultural “things”.  Not that morphology actually does recapitulate phylogeny (mostly disproven, but useful at times), however, a good analogy here is moving down mentally from adult to baby.  Encoding becomes less and less symbolic, vocabulary smaller and smaller, until eventually you wind up with literal encoding of basic drives.

With that said, there is almost no doubt that most calendars were based at least partly on changes in the seasons and stars, which was often the province of religion all the way up until science began to split from religion.  So, if we only consider the question “when did the Hebrew first develop their religion, and by consequence, their particular 7-day week”, then there is at least some kind of answer.  Orthodox Jews agree that the oldest portions of their scripture were not written before 1500 BC.  In fact, the oldest known small “fragment” we still have in physical existence of Hebrew scripture is dated to about 600 BC, and the oldest complete or nearly complete text (Dead Sea Scrolls) date to about 150 BC.  Now, consider for a moment that “Modern Humans” were first created about 200,000 years ago.  Certainly, that’s a whole lot of time to come up with theories as to why and how things work.  One of the biggest concerns of those early humans must have been the powerful natural elements, which seemed to have cycles.

It is logical to suspect people began to notice that there were big annual cycles, with cold and hot, but also smaller ones like night and day.  A next logical step would be to ask if there were cycles between these two?  Eventually people started to try and count things to know in advance when the cycle was going to repeat to thereby avoid death from not being prepared.  People create our own artificial categories in order to identify them and discriminate patterns, but nature seems to have some groupings too.  Together they combine into the ‘kinds’ we use in language which usually drift in meaning over time.  For most of history, religion was the ONLY means possible to understand the natural world, and many other things too, which involved super powerful forces over life and death.  Science was only a minor component integrated into most of the major religions as man started to organize into proto-states about 6,000 years ago.

As organization grew complex, consolidation or functional collaboration of religions became a key factor of success or failure to a civilization.  If there were incompatibilities, and no clear winner was possible, the civilization stagnated (or even deteriorated), and meaning sharing was difficult.. making progress such as in technology and social endeavors slower or even distracted towards the conflict.  The Christian religion is a direct descendant (and has surpassed in many regards) the older Hebrew religion, and some others which it absorbed to lesser degrees. This supreme dominance has had some influence on the adoption of the Christian calendar by the many other civilizations which previously used other ones (or which continue to use them, with less and less prevalence), although to what degree cannot be segregated from the impact of imperialism and other factors.  Today, I would argue that economics and technology drive this more than religion.