How early can you purchase tickets and reservations to: Accademia in Florence, Borghese in Rome, and The Vatican, online with a credit card? I am going to be there at the end of September 2017.
you can start looking now. if their calendars are not open for that month the web site will tell you. changes are difficult so make sure your calendar is set.
Most sites in Rome book about 60 days ahead of time. For the Vatican, be sure your credit card has the "verified by visa" protocol or it will be declined. This has been an issue now for about a month. Call and ask for the fraud department. Have them institute the "verified by visa" protocol and let them know when and where you'll be traveling. Also, let them know you'll be making online reservations before your trip.
Donna