You can check out the xmas-holiday-schedule here: http://www.subte.com.ar/horarios/horarios.asp
you are looking at the last column as that's the schedule for Sunday/Bank holidays.
Unfortunately we will never know if it is safe or not to travel at nite on the subte as the last train usually leaves at 10:30pm...taxis are pretty cheap though, so your best option is to go back home by taxi!