Please specify for each train you want to book: route, date, departure time. Otherwise we can only guess.

Indeed, without details. We cannot help. 24th of decembre is Christmas Eve, the 25th is Christmas. Some countries have the 26th as holiday  too.

A lot of public transport companies have a heavily reduced timetable then!

The days before and after are usually very popular travel days, so you might want to take into account very crowded long-distance trains, with some connections being fully booked. 

In Italy 24th is normal timetable, maybe at the evening some local transport is cancelled. 25 and 26th is like on sunday.

Details:
24th December
Geneve to Milano Centrale (EC39) 13:39 to 17:40
Milano Centrale to Roma termini (FR9653) 17:58 to 21:10

I dont have an option with the dates. 

Thanks!

These can be booked via tickets.oebb.at; they both have availability. You need to book them separately, otherwise it doesn't always show availability in both trains.

Also, do NOT use the option "Seat reservation only” but add the Interrail/Eurail discount and then look for normal tickets. Price is €11 for Geneva-Milan and €10 for Milan-Rome.

FR9653 leaves at 18:00. You might also consider the trains that leave at 18:10 or 18:30, so your connection isn't very tight.