exactly, zebo, you could equally be asking "why 6 weeks?" or "why 6 months?"
Re the days in the week: there were seven heavenly bodies known at the time. So Sun day, Moon day, Mars day, Mercury day, Jupiter day, Venus day, Saturn day. The middle ones got switched to Norse deities in English but you can still see them in French: mardi, mercredi, jeudi, vendredi.