Today I Found Out has a video explaining why we have 7 days in a week.
Two of the earliest known civilizations to use a seven day week were the Babylonians and the Jews. The Babylonians marked time with lunar months and it is thought by many scholars that this is why they chose a seven day week (though direct evidence of this being why they did this is scant). That being said, each lunar month was made up of several different cycles—on the first day, the first visible crescent appeared; on approximately the seventh, the waxing half-moon could be seen; on approximately the fourteenth, the full moon; on approximately the twenty-first, the waning half-moon; and on approximately the twenty-eighth, the last visible crescent. As you can see, each notable cycle is made up of about seven days, hence, the seven-day week.
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