I will arrive at CDG on a Friday and would like to buy the weekly Navigo pass from Friday to Friday. Can I do that? Or are they only sold as a one week pass from Monday to Sunday -- in which case I would have to buy two?
Navigo pass is only from Monday till Sunday. On Friday you cannot buy a Navigo pass for the same week, only for the following week.
I'd recommend you buy a single RER ticket from CDG to Paris, plus a Paris Visite 3-day, 1-3 zone for Friday, Saturday Sunday, then you can buy a Navigo week pass on Sunday for the Monday till Friday in Paris and you can also use that Navigo to go to from Paris to CDG Airport by Train.