Last time I was in Paris a week's metro pass started on Sunday and ended midnight Sat. Anyone bought a metro pass there recently. We'll be in Paris 6 days starting on a Saturday.
Bought a Paris Visite pass in November. It was not based Sun to Sat. Mine was a day pass, and it covered the day I used it. The Paris Visite cards can be bought in several day increments (I think it was 1, 3, 5, or 7). I believe you can buy them ahead of time for a specific set of consecutive days (if you Google Paris Visite, I'm sure you'll find more info.)
Just had a quick look. The pass starts the day of use, and is good for consecutive calendar days. There apparently is no 7 day pass as I mentioned in my previous post. Here is the link:
The "carte orange hebdomadaire" or weekly transit pass, starts on Monday and ends on Sunday. They're 16.30 Euro for zones 1-2 (the tourist areas), or 32.10 for zones 1-5, which includes CDG, Orly, and Vesailles. Compare that to round trip CDG-Paris 16.40 Euro), round trip Versailles (5.60), and a "carnet" 10 Metro tickets at 11.10 Euro.
The reason that Tom didn't find the Carte Orange Hebdomadaire that Norm describes on the English version of the RATP site is that it isn't listed there. Nor is the daily Mobilis pass or the Ticket Jeunes. They only appear on the French version of the site.