Unlike years and months, which use seasons and, in some cultures, the phases of the moon as a reference point, the days of the week is completely arbitrary.
So how did we get 7 days in a week? Why are there 7 and not 6 or 8? Which cultures were the first to adopt it and how did it spread to a point when almost everyone in the world today follows this weekly cycle?
More can be written, but you might like the FAQ section "Weeks, weekdays, and weekends". There are a number of aspects touched on: how did the seven-day week arise and get standardized? what alternatives are there? how did names of days arise? how did "week ends" arise?
This is not to discourage discussion. More questions, data, and debate are always welcome.