If you’re buying a rail pass for your trip, ask when you buy that about making train reservations – most places that sell European rail passes offer the service for an additional fee and then you can take care of it in one transaction. Alternately, you can always make reservations once you get to Italy. In your case, I’d advice going to a train station in whatever city you arrive in and booking yourself onto the May 1 train you want to take. You can do it ahead of time at just about any train station in the country, regardless of whether you’re going to be leaving from that station or city. In other words, you can be in Rome when you want to book your ticket from Florence to Venice.
I hope that helps; please let me know if you’ve got any other questions, and have a great trip!
