Simon Whistler explains the reason why modern society uses seven-day weeks in an episode of Today I Found Out. The episode looks back at the earlier cultures that laid the groundwork for the practice of seven-day weeks by basing their weeks on lunar cycles.
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.