“You are allowed to keep Shabbos until Tuesday night. They say the Amshinover Rav did/does that.”
MDG: “I never understood tht until I heard the following: Souls in Gehinom get a reprieve for Shabbat. Their reprieve lasts until the last shul says havdalah.”
Good point! I hadn’t thought of that. I heard a story of some Rav who always gave a shiur at Shalashudes time and he would make sure it lasted a while for that reason. When someone from the shiur was nifter, he said that it’s a good thing he participated in the shiur since he will get less gehinnom now.
I think it’s supposed to be a good idea to keep Shabbos as late as one can for that reason.