We have Eurail passes but plan some trains the require reservations. Is a day or 2 beforehand enough time to purchase them?
What country or countries?
Italy and France. Milan to Nice and Aix en Provence to Geneva.
You do not say when you are traveling.
That is not enough time. Those are popular routes.
Book early for best price and seat selections
France has capacity controls on railpass seats on at least some trains. That means the train might have a lot of vacant seats but none available to pass holders. I think to be safe you'd need to make your reservations more than one or two days ahead. I do not know when would be soon enough, but I guess that would depend partly on how many of each day's trains you would consider acceptable.
The best thing to do is not buy a Eurail Pass.
Aix-Geneva TGV: You should reserve further ahead to be sure of getting it. Easiest is to do it at the same time that you buy the pass. If you already have a pass, then go to https://ricksteves.raileurope.com/us/rail/point_to_point/triprequest.htm.
Milan-Nice Direct EC: The direct trains departing at 11:10 and 15:10 are operated by Thello and are not covered by rail passes. Other departures with a connection at the border, such as the 9:10 departure, are regular domestic Trenitalia trains, as below.
Within Italy: Faster domestic trains in Italy require reservations but don't place any artificial limit on the number of pass holders. It's often possible to reserve right up to the day of travel.