I was able to book my high speed train trip 3 months in advance last year when I visited.
One thing to know, in case you don't, 15 August is a bank holiday - Ferragosto. (some business were closed for most of the month in Rome.. wasn't a big deal,..) we flew back that day and were told to make sure you book the ride to the airport days in advance. Even the train had a different schedule. I was told it was due to the holiday, so just FYI.
Have fun!!!