How come every culture on the planet has a different calendar, yet follow the same system for a week?

I know it sounds a bit like a silly question. Every culture in the world follow a different Calendar, for instance:

1. Christians follow a solar calendar following the birth of Jesus Christ (2014. AD currently running)

2. Muslims follow a lunar calendar (Hijri) based on the birth of Prophet Mohammed.

3. Hindus follow a luni-solar calendar called Vikram Samvat (among many other versions).

Now the number of months and days in a month are obviously different in each of this calendar. However, how come all calendars happen to agree on the seven day week system (For example, today is a Friday in all calendars) ?

Looks like it should be obvious, I know. But still what makes it possible that they agree on the week and still differ in various other calculations?

• See: en.wikipedia.org/wiki/Seven-day_week It looks like mainly a religious significance. But, the Han dynasty of China had a 5 day week. – Alex R. Oct 31 '14 at 16:38
• There are also "weeks" based on 6 and 10 days intervals, so this is a matter of cultural convergence, not a math topic. – hardmath Oct 31 '14 at 16:40
• Reading point 4 here en.wikipedia.org/wiki/Week#.22Weeks.22_in_other_calendars one realizes that not only not all ancient "weeks" had seven days but there were cultures without division of their calendar by weeks. – Timbuc Oct 31 '14 at 17:38
• This question is based on a false premise that would have been uncovered by preliminary research – Mark C. Wallace Nov 3 '14 at 14:36
• The igbos are ancient Israeli Hebrews They Use four day cycle in a week seven times a month Its a version of abrahamic seven day week four times in a lunar cycle. H j nduka – H j nduka Jan 21 '19 at 8:41

We have an enormous amount of evidence for the ancient Babylonian calendar, but no evidence at all for a seven-day week in ancient Babylonia.

In the ancient world there were two forms of the seven-day week. First, the Jewish week (eventually adopted by Christians and Muslims) has numbered days from one (Sunday) to six (Friday) and the Sabbath on the seventh day. Although the Sabbath is frequently mentioned in the Old Testament, the use of the numbered week for dating is not attested before the first century BC, first with Jews, and then in the New Testament and other Christian texts. Second, the planetary week, where each of the seven days is named after a planet (Sun, Moon, Mars, Mercury, Jupiter, Venus, Saturn) is based on the astrological doctrine of the Lords of the Hours and the Lords of the Days; it is alluded to by the Roman poet Tibullus in the 1st century BC, then fully developed by the astrologer Vettius Valens in the 2nd century AD.

The seven-day week spread with Christianity and (specifically in India) with the reception of Greek astrology, beginning with the Yavanajataka, where the week is explicitly described as a Greek thing.

There is currently an ongoing research project on this question:

Pingree’s translation of the Yavanajataka (see in particular section 77):

• So the Jewish week was borrowed from the Greeks? – Anixx Nov 3 '14 at 14:38
• No. I said that there are two different forms of the week. – fdb Nov 3 '14 at 15:59
• The Greeks and Romans observed an eight-day week; the seven-day week wasn't fully adopted until the 4th Century. I remember hearing (alert! hearsay) that the observance of Sabbath by the Jewish merchants and bankers prompted adoption of a seven-day week by the rest. – Paul Rowe Aug 11 '15 at 14:19
• @PaulRowe. The Romans had an eight-day market week. Otherwise, I do not know what you mean by "fully adopted". The week was fully institutionalised with the adoption of Christianity at the time of Constantine. – fdb Aug 12 '15 at 19:00
• What does the following sentence mean: "Although the Sabbath is frequently mentioned in the Old Testament, the use of the numbered week for dating is not attested before the first century BC"? Do you mean that there's no record of anyone saying something like "the first day of the week" until the first century BC? If so, I'm not sure how that relates to determining day to day use. For example, even in our modern times, it's rare to note a date occurred on a Monday or whatever, unless it's contextually important. – 1973 Jan 9 '19 at 20:03

The seven day week is not necessarily universal, although it has spread through most of the world as certain cultures have dominated the globe. The Romans and then the Christians more or less pressed certain cultural elements on everyone.

Why 7 days? The easiest explanation is probably that there are roughly 28 days in a lunar cycle and so, dividing that into four equal "weeks" would yield roughly 7 days.

The earliest known 7 day "week" I believe was used in Babylon, on which a religious celebration took place every 7 days. However, this celebration started on the new moon. Hence, 7 day weeks, to make 4 celebrations per month.

• It’s an easy explanation, but it isn’t well-supported by the evidence. On the whole a religious motivation seems more likely. – Brian M. Scott Nov 1 '14 at 7:29
• Religion is part of culture, and culture is a product of environment. See my update. Yes; it was religious, but the religious aspect was derived from astronomical events. – Daniel Goldman Nov 1 '14 at 13:33
• It’s by no means certain that our seven-day week is derived from the Babylonian one, and I stand by my objection to your second paragraph. – Brian M. Scott Nov 1 '14 at 21:11
• The present English names of the days of the week aren’t terribly relevant, since they go back (via substitution of Germanic ‘equivalents’) only to the Roman planetary system of nomenclature, which is substantially younger than the seven-day week itself. (This is something that I’ve actually studied, but it’s off-topic, so I’ll stop here.) – Brian M. Scott Nov 1 '14 at 21:45
• People keep alluding to the lunar cycle as a cause, but no one is citing it. Even if so, that's a bit of a proto-explanation, I would think. Maybe the moon inspired a weekly schedule, but without any historical evidence, it's only supposition on what pre-history humanity was thinking. – 1973 Jan 9 '19 at 20:05

The solar year and the lunar month (and the solar day) are obvious, natural cycles of time. Since none of them is an integer multiple of another, a wide range of calendars have been created to reconcile them. They tend to still be in use because they've got a great deal of inertia (and often, religious tradition) behind them.

There are no natural sub-month cycles. The Abrahamic seven-day week is just one that's been in use; various other cultures have cycles ranging from the four-day market cycle of the Igbo to the Aztec/Maya 13-day ritual cycle. The domination of the Abrahamic week is probably simply because a third of the world's population follows it for religious reasons; none of the other sub-month cycles has anywhere near that following.

• "There are no natural sub-month cycles." What about the 24-hour cycle? – user2110 Apr 27 '15 at 5:08
• @RickyDemer. The day is a natural phenomenon. Its division into 24 parts is however artificial. – fdb Aug 12 '15 at 19:02
• People keep alluding to the lunar cycle as a cause, but no one is citing it. Even if so, that's a bit of a proto-explanation, I would think. Maybe the moon inspired a weekly schedule, but without any historical evidence, it's only supposition on what pre-history humanity was thinking. – 1973 Jan 9 '19 at 20:07

I guess, people look for fixed cycles in nature. the cycle of the moon, the wandering of the shadow on its surface takes 28 days, divide the horizontal axis of the moon into 4 equal parts you get for each part 7 days.

• True, but only if you do not count the one or two days when the new moon is not visible. The true synodic month is a bit more than 29½ days. – fdb Nov 4 '14 at 20:32
• People keep alluding to the lunar cycle as a cause, but no one is citing it. Even if so, that's a bit of a proto-explanation, I would think. Maybe the moon inspired a weekly schedule, but without any historical evidence, it's only supposition on what pre-history humanity was thinking. – 1973 Jan 9 '19 at 20:07