A Straight Dope Classic from Cecil's Storehouse of Human Knowledge

Why are there seven days in a week?

May 18, 1973

Dear Cecil:

Why are there seven days in a week? (Note: this question is not easy. Every other time division is based on natural phenomena but the hour/minute/second group.) The question, expanded, is this: (a) Why have weeks at all? (b) And if weeks, why seven days?

Cecil replies:

Thanks, John, but I'll decide which questions are easy, if you don't mind. Is this a test or something?

The most primitive calendar-keeping peoples had no weeks, but as civilization progressed it became apparent that a time period longer than a day and shorter than a month would come in real handy for scheduling certain activities. The primitive equivalent of shopping, for instance. The first "weeks," in fact, appear to have been intervals between market days. The intervals varied — some West African tribes had four-day weeks, and the Egyptian interval was ten days. The first Roman "week" was the nundinae, nine days counted inclusively from one market day to the next.

Many reasons are given for the seven-day week — probably it's the result of several of the following factors taken together: the four phases of the moon are roughly seven days in length; the Babylonians believed in the sacredness of the number seven; in ancient times, seven planets (including the sun and moon) were thought to exist (and indeed the days of the week were named after them). The Mosaic sabbath defined a seven-day period, and the dispersed Jews who observed it may have influenced the establishment of the seven-day week — it first appears in the early Christian era.

The present custom of a five-day work week plus a two-day weekend is a recent invention. The Sabbath (Saturday if you're Jewish, Sunday if you're Christian) was the only traditional day of leisure from the time of Creation (when, of course, God rested) until the mid-1930s. A "share the work" sentiment during the Depression resulted in codes of fair competition that established the 40-hour, five-day work week. The National Industrial Recovery Act of 1933, under which these codes were adopted, was later declared unconstitutional, but the practice remained. In 1938 the Fair Labor Standards Act renewed the 40-hour week's basis in law by stipulating that hours worked in excess of 40 were to be compensated at one and one-half times the normal rate.

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