While years and months are based on natural phenomena, the concept of a "week" is not. While there is some relationship to the lunar month (7 day weeks divide the month up into quarters) there have been many societies that did not use 7 day weeks as methods of dividing up time. It's only because of European dominance in the age of globalization that the 7 day week has become universal.
And the European week comes out of the myth of the Jewish bible.
This means that we divide our time up the way we do because some tribesman thousands of years ago told a story about God creating the world in seven days, and other people thought it was neat and passed it down, and it got collected into the "official" version, and, well, here we are.
Why did that tribesman choose 7? Were they already using seven day weeks as a way of dividing up the lunar month? Was he missing 3 fingers? Was he just drunk off his ass and picked a number at random?
The way individual actions or decisions filter down through history is fascinating and scary all at once.