First of all, thank you to anyone who replies. I really appreciate it. My question is this: My wife and I have a 3 day France Saver Pass...and I've noticed on all of the day trips we want to take (Paris - Stras, Paris - Avignon, Paris - Lyon), the TGV is the best way to go...and that one supposedly requires a reservation. How the heck do I make a reservation with also having to pay again for the ticket? And a side question: Am I correct in thinking that when we go to Strasbourg just for the day, and return that night - that will be counted as 1 day. Thanks guys so much!
Seat reservations are required on all TGVs. If you buy a regular ticket, they are included in the price of the ticket. If you have a railpass, you will need to buy a separate seat reservation for each train that requires one. They cost 3-4 euro and you can buy them at any SNCF station in France. Yes, if you go Paris-Strasbourg-Paris on the same day, you will use only one pass day; but each of you will have to buy two reservations.
TGV reservations really do not need to be purchased in advance, especially for just two of you travelling. As has been said, they can be purchased from any SNCF station. If you want to, you can purchase all the reservations at the first train station that you get to.
A day of travel on a railpass typically means 24-hours - from midnight to midnight. As long as all your travel is done within that time, there is no concern. Trains late at night are still considered same-day if they go past midnight, so long as the train started after 7 pm. Since you are travelling by TGV, I doubt that you will have any days of travel after midnight, so yes two trips in one day is still just one day on the railpass.
There are "SNCF Boutiques" around Paris where you can deal with train tix and reservations. Or, any train station will do. Make your reservations as soon as you can commit to a date; sometimes, especially around holidays, reservations aren't available for the date you want. I had this problem during the Easter holiday season in France a couple of years ago. I had to choose a different date/train.