Why humans chose the 7-day week

It was another Sunday but, being retired and in these Corona-times, the days of the week are merging into each other and are difficult to tell apart. My thoughts turned, again, to when and how and why the seven-day week was invented. While the primary purpose of the “week” today is to define a recurring separation between days of rest and days of labour, the week is also used to organise many other recurring human activities. It occurs to me that the reasons for inventing an artificial, recurring period of a few days, shorter than a month, must be based on

  1. the occurrence of periodic and repetitive human activity within a society, and
  2. the need to organise and plan such activity

Both these requirements precede, I think, the choice of 5 or 10 or 6 or 7 days as the length of the period. The need to have a period shorter than a month must come before the choice of length of period. The most important function of the period is now to identify periodic days of “rest” from days of labour. It seems that even in prehistory, in predominantly agricultural communities, this separation of days of rest from days of labour was important. In the past, rest-days were often also days of regular and organised worship. Social traditions built up around these periods (meeting family and friends and congregations of society members). Working practices during industrialisation adapted to weekly cycles. Organised sport today depends existentially upon the regular, repeating days of “leisure”. If the length of this period, as a sub-period of the lunar cycle, were to be chosen today we would be faced with the same limited choices that humans faced perhaps 12,000 years ago. The practical choice lies only between 2, 3, 4, 5 or 6 sub-periods of the month, giving respectively weeks of 14/15, 10, 7, 6 or 5 days. (The Romans, for a while, used an 8-day week, but such a week is out of step with months, seasons and years. An 8-day week has nothing to recommend it). The days within each period needed to be identified separately so that tasks could be allocated to specific days. It is probably just the difficulty of remembering 14 or 15 specific weekdays which eliminated the choice of half-monthly weeks. It would have been entirely logical to choose 10 days in a week (and would have had the added advantage of very easily naming the days after numbers). A 5-day week would also have had a natural logic. In fact, 5-day and 10-day weeks have been attempted at various times but have not caught on. For some reason(s) the 7-day week has been the most resilient and now its global domination is unchallenged. But the compelling reasons for choosing seven day periods are lost in the mists of history.

A quick search revealed that I had written about this 7 years ago:

Another Sunday, another week — but why?

There are no discernible periodicities that we have been able to find outside ourselves which take 7 days. There are no periodicities within ourselves either that are 7 days or multiples of 7 days.  There are no celestial or astronomical cycles in tune with 7 days. There are no movements of the sun or the moon or the stars that give rise to a 7-day period. There are no weather or climate phenomena that repeat with a 7-day period. There are no human behavioural patterns that dance to a 7-day tune. There are no living things that have a 7-day life cycle. (There is a branch of pseudoscience which claims that living cells may be associated with a weekly or a half-weekly cycle – a circaseptan or a circasemiseptan rythm – but this is still in the realms of fantasy).

It would seem logical that our ancestors must have first noted the daily cycle long before they were even recognisable as human.  As humans they probably then noted the lunar cycle of about 29 days and the yearly cycle of about 365 days. Our distant ancestors would also have noted that the period of the yearly cycle was a little more than 12 lunar cycles. By about 35,000 years ago we have evidence that the lunar cycle was known and was being tracked. This evidence is in the form of a tally stick with 29 marks – the Lebombo bone.

The invention of the seven-day week can best be dated to be at least 5,000 years ago to the time of the Babylonians. It was certainly long before the Old Testament came to be written to fit with the 7-day week which had already been invented and established. The story goes that

the seven-day week was actually invented by the Assyrians, or by Sargon I (King of Akkad at around 2350 B.C.), passed on to the Babylonians, who then passed it on to the Jews during their captivity in Babylon around 600 B.C.  The ancient Romans used the eight-day week, but after the adoption of the Julian calendar in the time of Agustus, the seven-day week came into use in the Roman world. For a while, both the seven and eight day weeks coexisted in the Roman world, but by the time Constantine decided to Christianize the Roman world (around A.D. 321) the eight-day weekly cycle had fallen out of use in favor of the more popular seven-day week.

The idea that the 7-days originates from a division of the lunar cycle into 4 seems improbable. The lunar cycle (synodic period) is 29.5305882 days long. Three weeks of 10 days each or five 6 day weeks would fit better. That the annual cycle of 365.2425 days comes to dominate is not so surprising. Our calendar months are now attuned to the annual cycle and have no direct connection to the lunar cycle. But it is our 7 – day weeks which remain fixed. We adjust the length of our months and have exactly 365 days for each of our  normal years. We then add an extra day every 4 years  but omit 3 such extra days in every 400 years to cover the error. We make our adjustments by adding a day to the month of February for the identified leap years but we do not mess with the 7 days of the week.

It is far more likely that the 7 days comes from the seven celestial objects visible to the naked eye from earth and probably known to man some 5,000 to 10,000 years ago. They were familiar with the Sun, the Moon, Mars, Mercury, Jupiter, Venus, and Saturn by then. Naturally each was a god in his own heaven and had to have a day dedicated just to him/her/it. The same 7 celestial objects are used for the days of the week not only in the Greek/Roman Western tradition, but also in Indian astrology. The Chinese /East Asian tradition uses the Sun, Moon, Fire, Water, Wood, Gold and Earth to name the seven days of the week. But this must have come after the 7 day week had already been established elsewhere. (For example, to name up to 10 days they could just have chosen to add days named for the Air, Beasts, Birds ….). Some languages use a numbering system and some use a mixture of all of the above. Rationalists and philosophers and dreamers have tried to shift to 5 and 6, and 8 and 10 day weeks but none of these efforts has managed to challenge the practicality or to dislodge the dominance of the seven-day week.

And now the whole world lives and marches – socially, culturally, politically – to the inexorable beat of the 7-day week.

The seven-day week must have started earlier than 5,000 years ago. We must distinguish, I think, between the need for first having such a period and then the selection of the number of days in such a period. The invention of names must have come after the selection of the number of days. The 7-day period must already have been in use before the Sumerians and the Babylonians got around to naming the days.

The need for such a period must have come in the Neolithic (c. 12,000 years ago) and after the advent of settlements with substantial populations (cities). Human hunter-gatherers (and even their forebears) would have followed the annual cycles and the seasons and would have been subject to the vagaries of weather. The availability of moonlight and the lunar cycle would have been important and well observed and was something well known by 50,000 years ago. But hunter-gatherers with their semi-nomadic life style lived in small groups of perhaps 30 or 40 and conceivably up to a hundred people. There would have been no great need for such groups to invent a sub-period of a season or a lunar cycle. The numbers would not have been large enough to warrant the invention of a “week” to help organise repetitive tasks. 

The Neolithic brought population density and specialisation. Carpenters and masons and spinners and weavers performed their specialities for many different projects simultaneously. The need to combine different specialist functions towards a goal was the new model of cooperation. Houses had to be built using a variety of specialists. Their labour needed to be planned and coordinated. I can imagine that the need to be able to plan work from different sources for the same day became critical. To be able to tell everyone to do something on a Thursday needed the Thursday to be invented. 

It seems obvious that increasing population density and specialisation generates the need for defining a “week”. But it does not answer the question of why 7 days? I can only speculate that human physiology comes into play. Physiology and nutrition of the time must have determined that labouring for 9 days of 10 was too much for the human frame and that resting one day in 5 was considered too idle. (Of course, nowadays with 2 rest days in 7 there is far more leisure time than with 1 in 5). I speculate also that the choice of an odd number of days (7) rather than six days comes from a need to define a mid-week day. I suspect the priests of that time had to have their say and therefore the day of rest was hijacked for worship and support of their temples. They probably could not overrule the economic necessities of the time and take over any of the other six days of labour. They still had a go, though, by naming some of the other days after their gods.

Perhaps the choice of 7 days was the first example of implementation of workers’ demands.


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One Response to “Why humans chose the 7-day week”

  1. AndrewZ Says:

    Very interesting. The fact that a 7-day cycle doesn’t correspond to any natural cycles might be part of the explanation. That means it can’t get confused with anything else, and won’t compete with any other cycle that already has its own symbolism and social functions. It allows “the week” to have a distinct purpose and identity of its own which is not linked to anything else.

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