Haven't been to Paris in years, regrettably. See to recall buying things like Formule 1, Carte Hebdomadaire, Coupon Orange, etc.

I'll be arriving on a Friday and staying through Tuesday.

If I buy Navigo, will I have to pay again on Monday or did they do away with that rule and run a week as seven days from purchase?

Otherwise, is the best bet a carnet? It seems like the Paris Visite is not a great deal.