I am arriving to Paris Saturday evening (CDG airport) and staying till Thursday afternoon (departure also from CDG). I thought week pass is an option but it appears I can't buy a week pass after Thursday midnight. I am quite confused. What would you recommend?
Navigo is now available as a day pass in addition to its more common week pass setup.
On Saturday, buy a Navigo card (5 euros) and load it with a week pass for the following Monday-Thursday and in addition add a Navigo day pass for Saturday 1-5 zone (on the same Navigo card you just purchased).
You can then use your Navigo card for traveling from CDG Airport to Paris by RER B train and throughout Paris, Ile-de-France area that day for an unlimited number of rides.
On Sunday you can add another day pass onto your Navigo card, only 1-2 zones this time (Central Paris) unless you want to see Versailles (zone 4) or other stuff, then you'll need more zones. Navigo cards cannot hold more than a single day pass on it at a single time, so you cannot buy both Saturday and Sunday day passes upon arrival at CDG Airport on Saturday.