I have no idea but I wouldn't expect many changes from one week earlier, for which the timetable can be seen on https://www.trainose.gr/en/.

Bestway is to ask TrainOSE(GreekRail) it looks currently like there are no train services on that day.
http://www.trainose.gr/

Well, OSE does not show any timetables for 15 May and later dates, so it looks like they still have to load their timetables.

rome2rio (even though it states there is 1 train/day and then shows 3 or 4-but all with changes in Paleofarsalos) says tickets are from 10-19€, which is about right for current GR pricing so a passday would cost a lot more for just this trip.

There is-its Gr after all, also the KTEL bus, price probably more or less same level.

IF in ATH try to go to station a day or 2 before and ask (and hope for a competent worker)