# 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.
Commented Oct 31, 2014 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
Commented Oct 31, 2014 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
Commented Oct 31, 2014 at 17:38
• This question is based on a false premise that would have been uncovered by preliminary research
– MCW
Commented Nov 3, 2014 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 Commented Jan 21, 2019 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? Commented Nov 3, 2014 at 14:38
• No. I said that there are two different forms of the week.
– fdb
Commented Nov 3, 2014 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. Commented Aug 11, 2015 at 14:19
• And ancient dates often aren't exact at all, sometimes just giving the year in some king's reign. Like an old man reminiscing. But a note to a servant along the lines of "finish the job by Tuesday" would be pretty great evidence that may exist.
– user1973
Commented Jan 9, 2019 at 20:10
• The Romans did have an 8 day market week. On the other hand, you could take the Kalends, Nones, and Ides of each month as a way of dividing each month into unequal weeks for political purposes. Commented Jan 21, 2019 at 17:07

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
Commented Nov 1, 2014 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. Commented Nov 1, 2014 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
Commented Nov 1, 2014 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
Commented Nov 1, 2014 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.
– user1973
Commented Jan 9, 2019 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
Commented Apr 27, 2015 at 5:08
• @RickyDemer. The day is a natural phenomenon. Its division into 24 parts is however artificial.
– fdb
Commented Aug 12, 2015 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.
– user1973
Commented Jan 9, 2019 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
Commented Nov 4, 2014 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.
– user1973
Commented Jan 9, 2019 at 20:07