That’s a very good and useful addition by Carol. The week pass on the Navigo always starts on Monday. If you buy it on a Monday, Tuesday, Wednesday or Thursday, it’s good for travel in that current week. Which means you can use it up to and including the Sunday of that current week.
If you buy it on a Friday, Saturday or Sunday, it’s validity will start on the Monday of the new week.
So let’s assume you arrive in Paris on Friday October 4 and you want to use the Navigo Weekly pass for Friday October 4, Saturday October 5 and Sunday October 6, you can purchase it starting Friday September 27 up to and including Thursday October 3.
Which means that the only way to purchase it, is before you arrive in Paris. This means that you must load it on a virtual Navigo Decouverte on your phone or smart watch.
I purchased a pass on a virtual Navigo Decouverte before I arrived in Paris and I found the process to be easy. Download the IDF Mobilités app, click on “purchase” and it will all explain itself.