Why does a week have 7 days?

The year lasts 365 days. This is the time the seasons need to repeat themselves. These 365 days are divided into 12 months. This division results from the orbital period of the moon, because the moon needs 28 days to go once around the Earth. In these 28 days, the moon goes through four phases (full moon, waning moon, waxing moon and new moon). Therefore, each of these phases requires 7 days. And that's our week. 

Even in ancient times, people have combined this interplay in the sky with the season - and the result was the week having a length of seven days.
2013-09-06 at 23:34

Once, I have heard that this division has a biblical origin. The Bible says that God has created the world in six days and the seventh day was a rest day. That would exactly correspond to the classification of Monday to Saturday and Sunday.

But maybe, it was just the other way around: First, there was the scale resulting from the moon, then there was the creation story based on this week basis.
2013-09-09 at 21:50

Although, today the 7-day-week is established, this does not mean, that other cultures had developed other calendar systems or that there were other ideas at certain times.

One example is the French Revolutionary Calendar after 1789, where one year had 12 months of 30 days, each consisting of 3 decades with 10 days. In this system, there was even the decimal time, wherein one day was divided into 10 hours with 100 minutes of 100 seconds. Despite, the Soviet Revolutionary Calendar, had a week that was only 5 days long (the Christian rest days should be abolished, every day should be used for working) and also the Mayan calendar had an entirely different concept.

As you can see, the "Seven Day Week" is not "God-Given". Rather, it is a simple agreement that has stood the test of time. Finally, the rhythm of 5 days of work and 2 days if free time is a good compromise, which is not too long and not too short for both workers and companies.
2013-09-08 at 20:37