Can I buy the pass several weeks in advance as long as I begin it my first day. We arrive in Paris on a Monday.
You can do that. Just load it when you get to Paris. But you can buy the kit and put it together any time.
Do you mean the Navigo Semaine (English: Navigo Week), formerly called the Navigo Descouvert?
Yes the one from Monday to Sunday
I dont think you can buy them that way any more. Last time we were in Paris we bought the kit at the station and the lovely lady at the service window even put it together for us. Took maybe 10 minutes. The link contains all the info you need to know: https://parisbytrain.com/paris-train-metro-week-pass-navigo-decouverte/
IMPORTANT: pay attention to the black out dates, when the ND will not be allowed: July 20 to Sept 8
Thank you for the link and replies. When I held the card to the phone reader it did show the pass as active but empty! I think they had just begun the picture ID and the chip on the passes.