The Week

It’s disputed, but it seems to be based mostly on Israelite and Biblical tradition. There is ancient evidence that the Jewish people acknowledged a seven-day week as early as 600BC.

There seems to be some argument whether seven days as any astronomical significance. Most time divisions are based on astronomy – a circuit of the Earth around the Sun (a year), etc. But what lunar significance does a week have?

An author of a book about the week says this:

The ancient Romans settled on a seven-day week around 2,000 years ago because there are seven celestial bodies that can be seen from Earth: the sun, the moon and the five planets.

But…

Also around this time, Jews were marking the Sabbath every seven days, with Christians later counting seven-day cycles

So the seven-day week seemed to exist in multiple cultures for multiple reasons.

This Stack Exchange question claims that it’s a subdivision of the lunar cycle.

An article at timeanddate.com also cities the Babylonian lunar cycle theory (and takes a shot at Biblical accuracy at the same time):

The Babylonians… rounded the Moon cycle down to 28 days and divided this time span into 4 periods of 7 days each, using leap days to stay in sync with the Moon phases in the long run.

This 7-day structure is also believed to have ultimately informed a number of popular creation myths, such as the Book of Genesis in the Hebrew and Christian Bibles

The most common cited source seems to simply be Jewish tradition, which points to the story of creation (God created the world in six days, and rested on the seventh) as the source.