Week

Ever wonder why 24 hours is a day, 7 days is a week, 365 days (or 12 months) is a year?

We experience day and night in 24 hours, hence a day is 24 hours.
There is a full moon in a month, and hence a month.
And there are four seasons in 365 days, and hence a year.

But what about a week? Why is a week 7 days? Why can't it be just 5 days, or 10 days? Why can't we have a system such that there are 12 months, and each months has different number of days (same as current system). We can do away with the days, and just use notations such as today is the 4th day of the 8th month; or the 31st day of the 12th month, etc.

The only reason I can think of as to why a week is 7 days is because God create the universe in 7 days (actually he used 6 days, and rested on the 7th). Well, anyway I did a quick search and found that I'm half right (actually there are a lot more different reasons than just one).

http://en.wikipedia.org/wiki/Week#Origin_of_the_seven-day_week