while the seven day week is arbitrary, it is also worth noting that the French Revolution had experimented with a metric ten day week, which ended up being a total bust because laborers only got one week in ten of rest rather than one in seven.
There's no reason you couldn't do off 2 on 8 or off 3 on 7 or off 2 on 2 off on 2 off on 2 or off on 3 off on 2 off on 2 or off on 4 X2 or...just about anything...
We see this today where people's schedule's aren't really in sync with the week where they'll do 4 on 3 off or split days off or 21 on 7 off (regardless of days in the month).
There are also a few people who do 6 28 hour days in a week claiming that it boost productivity because their days are longer.
If you've ever been in a position where you can do "weekend stuff" any day of the week, it can be noticeably better in many ways.