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Today it's Sunday. 7 days ago it was Sunday. 7000 days ago it was Sunday. But what about 70,000 days ago, 700,000 days ago, and 7 million days ago? Our current 7-day week cycle has not continued unbroken forever. So my question is, when did our current 7-day week cycle begin? Note that I'm not asking when any 7-day week cycle was invented, but rather the specific cycle we're still on.

Here is what Wikipedia says:

The continuous seven-day cycle of the days of the week can be traced back to the reign of Augustus; the first identifiable date cited complete with day of the week is 6 February AD 60, identified as a "Sunday" (as viii idus Februarius dies solis "eighth day before the ides of February, day of the Sun") in a Pompeiian graffito. According to the currently-used Julian calendar, 6 February 60 was, however, a Wednesday. This is explained by the existence of two conventions of naming days of the weeks based on the planetary hours system: 6 February was a "Sunday" based on the sunset naming convention, and a "Wednesday" based on the sunrise naming convention.

So February 6, 60 AD is the earliest identifiable date with a day of the week cited with it, but this was based on sunset naming convention, rather than the sunrise naming convention we use today. So what is the earliest identifiable date with a day of the week cited with it that is based on the sunrise naming convention?

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    "7000 days ago it was Sunday" This is possible, but if you have a source for it it would already be part of the answer I suppose.
    – Evargalo
    Oct 30, 2017 at 8:54
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    @DenisdeBernardy I want to know what is the largest value of x such that 7x days ago, it was considered (by someone or the other) to the same day of the week it is today. Today is Monday. What number of days which is a multiple of 7 can I go back in time where I'll find someone who will agree with me that it's Monday? Oct 30, 2017 at 12:42
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    @Evargalo It's obvious. 7000 days ago is just 19 years ago. I don't know when our current 7-day week cycle began, but I'm sure it began earlier than 1998 :-) And any multiple of 7 days ago, as long it's after the current 7-day week cycle began, would be the same day of the week as it is today. So it should be clear that 7000 days ago it was the same day of the week as it is today. Oct 30, 2017 at 12:45
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    The basque language (whose origin is still unknown) originally comprised only three week days: Astelehena, Asteartea, Asteazkena. We still preserve these days' names, to which at some moment in history the rest 4 days were added. So, I am looking forward to an answer. Just as curiosity, the meaning of the days are: - beginning of the week - middle of the week - end of the week Oct 30, 2017 at 14:09
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    Years ago I asked on a different forum whether we could be sure the Sabbath of Jesus of Nazareth's time was a multiple of 7 days from the Sabbath observed by Judaism today. Everyone seemed to feel it was but no hard evidence was forthcoming. Oct 30, 2017 at 15:38

3 Answers 3

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Timothy is somewhat correct with identifying Babylon, however the Babylonians only borrowed the system. Most of our time keeping dates all the way back to ancient Sumeria (2600BC-ish?) and is mentioned in the epic of Gilgamesh.

Earliest we can trace it is between 2600BC and 3000BC.

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    What's mentioned in the linked text are "seven day periods". It's not clear to me whether these were strung together to cover all of time. Apr 5, 2019 at 4:15
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    Yes, it seems there are two parts to this question. If different ancient civilizations were using 7 day weeks simultaneously (say), to what extent could they be interchanged meaningfully?... and, if as civilizations rose and fell, we assume the time system of the successor would be imposed on the vanquished, then it seems one system has eventually prevailed over all others, and is still with us today. Whose is it? Feb 6, 2020 at 16:37
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Those who believe in the strict truth of the book of the Bible (I do not myself) would of course say it goes back to God's creation of the world, see Genesis Chapters 1 and 2

Most others say the week comes from Ancient Babylon, known directly from at least 500 BC but presumably older, if the Jewish religion absorbed it during the Captivity in Babylon which is thought to have ended 538 BC see:

https://www.timeanddate.com/calendar/days/7-days-week.html

More information in Gerard Clarke's book 'Heirs to Lost Kingdoms'

The 7 day week's origins appear to be based on Babylonian Astrology, which knew 7 'planets' (i.e. wanderers in the sky, as opposed to the fixed constellations): Sun, Moon, Mercury, Mars, Venus, Jupiter and Saturn. Each was thought to 'rule' the first hour of a day of the week. This made each day propitious for some activities but not for others, although there was not originally a Sabbath or a Weekend.

Hence why several religions that later developed in the Middle East have one day of a 7 day week as their Holy Day, but possibly also why there is no consensus as to which day it is: Wednesday for Yazidis, Thursday for Druze, Friday for Muslims, Saturday for Jews, Sunday for Christians. [For some reason no religion seems to like Mondays!?]

This took longer to spread to Europe. The 1st Century BC Roman writer and politician Cicero complained that the Jews were 'lazy' because they refused to work 1 day in 7.

His contemporary the Roman general Pompey had an advantage attacking Jerusalem in 63 BC because the Roman besiegers worked 7 days a week to build up ramps from which to attack the city walls while the pious Jewish defenders would only work to knock them down 6 days a week and stopped for the Sabbath.

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    This doesn't seem to answer the question (which was clarified in comments).
    – user2848
    Oct 31, 2017 at 3:58
  • Babylon perpetuated the 7 days, but were not the originators of it.
    – Twelfth
    Nov 14, 2017 at 19:47
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    I'd suggest a lunar origin for the 7-day week. The moon's cycle - month - is pretty close to 28 days, which is too long for practical use. But you can easily fit 2 (fortnight) or 4 (week) periods in the month.
    – jamesqf
    Nov 25, 2017 at 18:21
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It seems probable to me that the answer to the question will be found by counting back from today through the Judaic calendar, down an unbroken cycle of days in the sequence Friday-Thursday-Wednesday-Tuesday-Monday-Sunday-Saturday.

Why the Judaic calendar?

There are three components to the question. The calendar must be:

  • as old as possible (obviously)
  • extant (still in use today, not one that has disappeared)
  • unbroken (this is the part of the question that seems to me to be missed out by the other answers: that it includes "the specific cycle we're still on")

Antiquity of the calendar

The Judaic calendar has been around since the 6th century BC at least.

It has existed longer than the calendar that started with the Romans: in AD 351 (according to Wikipedia) "Emperor Constantine officially decreed a seven-day week" across the Roman Empire, that included a Sunday as a public holiday.

It has also existed longer than the Chinese and Indian calendars, according to the evidence available.

In current use

There could be other calendars, that ran for longer - but are no longer in use. At any rate, the Judaic calendar is very much in use.

Part of an unbroken cycle

The week is of defining importance in Judaism. It seems like something that Jewish culture would hold on to with some vigour. It's possible that the thread were broken at some point in Jewish history, and had to be restarted, but I have never heard of that.

On the other hand in Babylon and more recently in Roman times the frameworks of Jewish culture were at risk, so if the cycle were broken that would seem a likely time.

After the Romans, the dispersal of Jews actually makes it easier to keep the calendar going - distributed calendar-keeping, rather than being at the mercy of a single, dominant other system. For example, if it turned out that Jews in North Africa and Eastern Europe thought a different day was the first day of the week upon encountering each other after a long period, that would tell us that a thread had been broken - but I haven't heard of that either.

Best guess

So my best guess would be to look for evidence in Jewish history, and that the unbroken cycle might go back beyond the Romans, but that the start of the thread is unknown - we don't have a Constantine in this case to give us a convenient decree.

But, the Babylonians made lots of decrees, and it seems possible that they adopted the Jewish calendar, so perhaps a firmer answer can be found there.

Complication

One complication is that "our" (for some value of "our", I realise that this doesn't apply universally) days start at midnight, while others use sunrise or sunset. But we could probably mostly agree at midday.

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