I know this must get asked all the time but the terms have changed since I last used my pass and I’m not very clear on how it will work for me. We arrive on a Saturday and depart on the Tuesday, So staying three days.
Can we buy the pass when we land at CDG and use it over the three days then for the journey back to CDG on the Tuesday or will it run out on the Sunday (the day after we arrive).
A Navigo purchased on Saturday is not valid for use until Monday start of service. So Navigo would not be a good solution for your trip.
Buying a single use Paris RER train tickets from and back to CDG airport + Paris Visite 3-zone 3-day would probably be the best balance of cost effective and easy.
Btw, your stay from Saturday till Tuesday is actually four days: Saturday, Sunday, Monday, Tuesday, so the Paris Visite wouldn't cover one of those days, depending on whether you start using it on Saturday or Sunday.
You can purchase all the train tickets upon arrival. Single-use Paris RER train tickets and Paris Visite don't have to be used on the same day of purchase.