OK, so why 7 days in the week? Because the bible said so?
Let us thank the Babylonians for this. There was really no natural reason for a seven days but the Babylonians broke down the lunar month into manageable chunks of seven days since the number seven had a mystical significance to them. They assigned 7 celestial bodies in each days of a week then decided to measure a full lunar cycle as 28 days and then divide that into 4 weeks of seven days each. Then, they made the seventh day as “holy” marking rituals every seventh-day.
The Hebrews adopted these days and so as other cultures in ancient Mesopotamia and throughout the ancient world, even in the Far Eastern empires of China and Japan. BTW, when the Book of Genesis was written in approx. 950 BC. the Jews have borrowed the idea of the 7 days from the Babylonians. The Lunar calendar with seven days a week was a Babylonian invention. The Hebrew "god" has nothing to do with it. His "story-tellers" just plagiarized the idea from Babylonian astronomers.
The Romans adopted the Babylonian system and added many features of the modern 7-day week. Roman Emperor Constantine formally adopted the seven-day week in AD 321, it had been in use informally since the first century BC. A Christian convert Constantine made Sunday - the Christian Sabbath - the first day of the week, and Saturday - the Jewish day of rest - as the last.