If it is the train that you board at e.g. 13:18 is the same train that you leave at e.g. 14:00 the following day, then only one travel day, the day of the departure of the train, should be used.
You don't need a new travel day as long as you stay in the same train after midnight.
You can even use any train on the dep. day and still board an overnight (there are simply no such long trains in IT, ist not India) to final destination as last. NOTE that even though some anti-covid rules are now relaxed in IT there still seem to be strict rules about using sleepers in overnight trains.
ALL IC and Frecce trains from trenItalia NEED to be reserved and this is best done locally at station-or maybe via app-or in advance via oebb.at -as has been told here som many times