The Seven-Day Week


Why was the seven-day week favored? Weekday Connotations Order of the weekdays Thank you all!


There have been weeks of different lengths in human history, but "The Christian, the Hebrew, and the Islamic calendars all have a 7-day week. ... There is no record of the 7-day week cycle ever having been broken. Calendar changes and reform have never interrupted the 7-day cycles. It is very likely that the week cycles have run uninterrupted at least since the days of Moses (c. 1400 BC), possibly even longer." (from the Calendar FAQ). The seven-day week dominated globally (at least in the 20th century).

There is an ongoing discussion about the origin of the seven-day week which gives interesting historical information and speculations. It seems that the week originated in Babylon or Egypt. Any new light shed on this question is appreciated.

Why was the seven-day week favored?

The common explanation is that the seven-day week was established as imperial calendar in the late Roman empire and furthered by the Christian church for historical reasons. The British Empire used the seven-day week and spread it worldwide. Today the seven-day week is enforced by global business and media schedules, especially television and banking.

The above explantion is an evolutionary (after-the-fact) argument: "the seven-day week just got the most followers, another week length could have gained world dominance ...". But why was the seven-day week chosen in the first place? In other words, did it mean anything to have seven days in a week?

Moon Phases

It is interesting that five-day and six-day weeks make a better short term fit (6 times 5 is 30) to the lunar (synodic) month (of about 29.53 days) than the current week (4 times 7 is 28). The seven-day week may have been chosen because its length approximates one moon phase (one quarter = 29.53 / 4 = 7.3825).

According to this theory, if the time from new moon to new moon were e.g. 31.08 days, we'd have an eight-day week (one quarter = 31.0 / 4 = 7.77 days).

The four-quarter system is not the only way to divide a lunation. Historically, there was at least one other way of dividing a lunar month:

Joseph Needham reports that the Chinese divided day (obviously midnight to midnight) and lunation into seven parts (full moon to full moon). Because this system (like the new-waxing-full-waning system) is a generalization of certain natural cyclical processes, this is also applicable to the week - the descriptions and hexagram associations are given in the table below.
(Joseph Needham, Wang Ling, Science and Civilisation in China, Cambridge University Press, Cambridge 1956, vol.2 History of Scientific Thought, p. 332, 13. Fundamental Ideas of Chinese Science, Table 17, Association of the kua with the lunar and diurnal cycles in the Tshan Thung Chhi)

A calendar that is built on the seven is the Seven Space calendar.

Solar Year

Does the seven-day week make a better fit to the solar year? On first sight: no, 365/5 is exactly 73, while 365/7 = 52 with rest 1. And using a four-year cycle of 3*365 + 366 = 1461 days doesn't help either: 1461/7 = 208, rest 5.

It is obvious that 7 days * 13 weeks make a "season" or quarter year of 91 days. Both factors are prime numbers (odd numbers were considered "good luck"). To get the 365-day year, make one season a day longer (or two longer seasons in leap years). The card deck of 4 suits with 13 cards each (plus the occasional joker) is a model of this (see astonishing playing card math or Karl Palmen's playing card calendar).

Explanation Attempts

The 7 was favored in the West and by the Chinese. Here are some explanation attempts for this:

I think the "seven-planets" explanation is the most probable, since the week is so old and the planetary code is still in the names. The Babylonians may have wanted to code knowledge into the number or honor all celestial gods equally (see Sevenfold Star).

More about the week

Other Calendrics at this site.

Essays on mathematical themes.

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