Hi Lek
it seams a bit strange when you look at the Jadrolinija schedules, the low season schedule ends apparently on the 30th of Mai and the high season table does not start before the 2nd of june.
I although think there must be a ferry leaving dubrovnik on sunday the 31th as the sunday ferries are both included in the low and high season table, only question is if ferry departures at 9 or 10 AM.
as this is very special case, i would recomend you to contact jadrolinija directly, to regarind ferry on 31th of may.
regards
Morten