Why does a week have seven days?
The short answer
The seven-day week is not forced by a law of nature. A rough division of the lunar month, ancient observations of the sky, and religious and social customs helped the cycle spread across many societies.
The connection with the Moon
The lunar cycle is about 29.5 days. Dividing it into roughly four parts gives periods close to seven days. The fit is not exact, but it offered a useful rhythm for community life.
Not a universal natural rule
Some societies have tried calendars with cycles of ten days or other lengths. The seven-day cycle became established through religion, trade, law, and work customs, so it is now used in much of the world.