Emily,

I took the Sleeper from Milano to Messina. I was able to board about thirty minutes before departure. I would suggest you go to Roma Termini and go to the ticket office and ask what time you will be... (More)

These are available for booking as of a couple of days ago!

I emailed on this.  They said that the summer schedules have not yet been issued by the train companies.  They expect them by month's end.