My friends and I are thinking about buying a Week Pass for our week in Paris, but we arrive on a Saturday. Is it possible to buy a Pass that works from saturday-saturday instead?
Not possible for Paris Navigo week pass to work from Saturday to Saturday. It strictly works Monday (00:00) to Sunday 23:59.
You could buy the Navigo card on Saturday, charge Navigo with a 1-5 zone day pass on Saturday to use it to get from CDG airport to Paris by RER train, then use rest of day, charge your Navigo card with another day pass on Sunday for 1-2 zones or however many you need, then charge Navigo with a week pass for Monday - Saturday, 1-5 zones, which will cover all of the Ile-de-France region, plus get you from Paris to CDG airport by RER train.