Where did the idea of weeks originate and why do they have seven days?

Probably from the Middle East

The Iranian calendars or Iranian chronology (Persian : ﮔﺎﻩﺷﻤﺎﺭﯼ ﺍﯾﺮﺍﻧﯽ , Gāh-Šomāri-ye Irāni) are a succession of calendars invented or used for over two millennia in Iran , also known as Persia. One of the longest chronological records in human history, the Iranian calendar has been modified time and again during its history to suit administrative, climatic, and religious purposes. The most influential face in laying the frameworks for the calendar and its precision was the 11 century Persian polymath , hakim Omar Khayyam . The modern Iranian calendar is currently the official calendar in Iran. It begins at the midnight nearest to the instant of the vernal equinox as determined by astronomic calculations for the Iran Standard Time meridian (52.5°E or UTC+03:30 ). It is, therefore, an observation-based calendar, unlike the Gregorian , which is rule-based.

Following the rise of extremist Christianity in Europe, Sunday was declared a holiday instead of a Friday.

_{Where did the idea of weeks originate and why do they have seven days?}

The idea of weeks, not necessarily seven days in length, originated independently, across many cultures, and at various times, ultimately for purely utilitarian reasons, such as having a practical, medium-sized time unit, between a day (which is oftentimes too short) and a month (which is oftentimes too long).

_{Where did the idea of weeks originate and why do they have seven days?}

The idea of seven-day weeks, specifically, seems to have originated in ancient Sumer, during the first half of the third millennium BCE.

_{Weeks, on the other hand, have no relationship to astronomical observations.}

Some don't, others do; thus, the Roman nundinal cycle constitutes a clear example of the former, whereas the Egyptian decan or the Dacian six-day week¹ are regarded as instances of the latter, representing either a third or a fifth of a lunar month, respectively.

¹ _{Yes, I am well aware that the linked post nowhere explicitly states that the 180 lanky pillars, corresponding to a semester, are periodically subdivided into groups of six, by thirty short, sturdy ones, (also) representing the number of days in a month.}

_{Why do we have them and where do the seven days come from, when it just as well could have been more or fewer?}

As already stated above, weeks containing both a greater and a smaller number of days did exist throughout history, and across the globe.

_{Why do we have them and where do the seven days come from, when it just as well could have been more or fewer?}

_{A cursory read of [encyclopedic] articles [...] reveals that weeks appear to originate in Mesopotamia, but none of them explain the number seven in particular. Is its origin unknown?}

That is indeed correct; specifically, in Lower Mesopotamia (particularly, Sumer), during the early dynastic period, as already stated above.

_{Where did the idea of weeks originate and why do they have seven days?}

_{Why do we have them and where do the seven days come from, when it just as well could have been more or fewer?}

_{A cursory read of [encyclopedic] articles [...] reveals that weeks appear to originate in Mesopotamia, but none of them explain the number seven in particular. Is its origin unknown?}

Though initially stemming from ancient Sumer, as already noted twice above, the seven-day week only came into widespread use due to the (direct or indirect) influence of the famous Babylonian empire on the Greek-Roman world of classical antiquity; thus, while not its original inventors, the Babylonians were ultimately responsible for not only preserving its continuous usage, but also facilitating it unto others; but why would they do that in the first place ?

Of course, part of the reason would be that humans, in general, perpetuate ancestrally inherited customs through the generations, unless they prove either completely unnecessary, or seriously detrimental, or until a significant improvement appears somewhere along the way, thereby altering the long chain of transmission. So, while tradition does certainly play an important part, what other reasons could there have possibly been, to help explain not merely its adoption and preservation, but also its proliferation ?

To better understand this, let us take the following factors under more careful consideration:

Babylonian mathematics

• Before asking ourselves why or how Babylonians counted days, in particular, let us inquire as to how they counted in general; thus, they are reputed to have employed a rather robust sexagesimal counting system:
  • Human beings have five fingers on each of their four members, hence the use of decimal and vigesimal counting systems, such as that of the Maya, for instance.
  • Apart from this, they also customarily divide things into halves, thirds, and quarters, hence the occasional use of a dozenal base.
  • The smallest numerical value whose mathematical properties satisfy all aforementioned divisibility requirements is sixty, which, rather unsurprisingly, and as a direct consequence of the above, possesses a relatively large number of divisors:
    • 1, 2, 3, 4, 5, 6, 10, 12, 15, 20, 30, 60.
  • Notice that the first or smallest number absent from the above list is none other than seven.

Babylonian astronomy

• The shape and position of the moon in the (night) sky repeat periodically, with a mean value of about 29¹/₂ days for a synodic month, and of 27¹/₃ days for a sidereal month, respectively.
• Throughout this time, it passes through four easily discernible lunar phases: empty, half-full, full, and half-full again, averaging between 6⁵/₆ days and 7³/₈ days, or, in other words, about seven days.

But how about other celestial bodies ?

• Indeed, almost all visible lights in the sky move in the same direction, the small number of exceptions known to the ancients being the seven classical planets,
• not to mention the important role played by the seven stars of the Pleiades across many ancient mythologies, explaining their presence within various cuneiform star catalogues, such as the MUL.APIN.

Having weeks of seven days is a direct consequence of the lunar cycle. Each 29 day lunar cycle comprises four equal phases of 7.25 days long each. Starting from New Moon:

• Waxing crescent ending in First Quarter;
• Waxing gibbous ending in Full Moon;
• Waning gibbous ending in Third Quarter; and finally
• Waning crescent ending in New Moon.

These seven and a quarter day cycles are readily apparent to all individuals, and comprise an obvious cycle greater than a single day and less than a full month.

In a society without electric street lights, this omnipresent cycle of bright nights under the gibbous moon alternating with much dark nights under the crescent moon is one that drives all evening activities. Groups would time their evening activities to occur under the gibbous moon, and as close as possible to Full Moon.

Note that the day of, and the two days just before and after, the Full Moon are nearly twice as bright as the evenings two days before and after the Full Moon. This albedo effect is nearly unique to dust covered surfaces, such as the lunar surface, and is how and why scientists were nearly certain the moon was dust covered long before the Apollo missions in the 1860's.

As for history: The Romans marked the days of each month relative to the Calends (day following New Moon), Nones (day following First Quarter); and Ides (day following Full Moon) of each lunar cycle.

Having these three brilliantly lit nights each month, under which one might even be tempted to try reading a newspaper, is absolutely unmissable to anyone who is frequently about at night away from streetlights.

For those city dwellers with no experience of this: treat yourself to a true nature experience. Attend a dark night sanctuary twice during the coming year, on a cloudless or nearly so night: once at or within a day of New Moon; and again at or within a day of Full Moon. The difference will astound you.

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Objections ae raised below that the structure of four seven day weeks in a month is not natural because, at various times and places, other subdivisions lengths of the lunar cycle existed.

That's horse hockey. The seven day cycle is the most common; but that doesn't have to mean it's the only one that makes sense. If a society wished or needed to distinguish the brightest third of the lunar cycle, around Full Moon, it would be natural to do that as well (or instead of) with three nine or ten day cycles. That doesn't invalidate the lunar origin of the seven day week. Being one natural read for such a cycle doesn't require being the only possible such read.