What is Sundaytreat?

This is a special thing that Jack and Ma request for Sundays. Old Nick seems to give in at least a little on this point. Some Sundaytreats are actually necessities, such as Ma's pain medicine. For Jack, Sundaytreat is a symbol of hope that he'll get some gift that's very special. It represents that same symbol for Ma, except her hope is that they'll get something that will make their captivity a little less horrible.