Let's start in the middle to answer this question about "A Day's Wait." In the last line the father and narrator says, "The hold over himself relaxed ...." This refers to an earlier passage:
'Your temperature is all right,' I said. It's nothing to worry about.'
'I don't worry,' he said, 'but I can't keep from thinking.'
'Don't think,' I said. 'Just take it easy.'
'I'm taking it easy,' he said and looked straight ahead. He was evidently holding tight onto himself about something.
The word "hold" refers to the emotional restraint the boy was bravely exerting in the face of (as he believed) advancing death. He was holding in fear, worry, perhaps panic; he was holding his dignity and self-control together. Therefore in the last line, the hold that relaxes is the hold of self-restraint in the face of humanity's greatest fear and darkest voyage.
The line preceding the last line is also important. It states that the boy's fixed "gaze" at the foot of his bed "slowly relaxed." This indicates that he was letting go of his courageous determination as the news sank in and he made the mental adjustment to the idea that death would not be his immediate fate.
The meaning of the last sentence will now be a easier to get at:
The hold over himself relaxed too, finally, and the next day it was very slack and he cried very easily at little things that were of no importance.
We already understand the "hold" that relaxed was his courageous determination to die with grace and not be hysterical about his fate. The next day, his nerves were spent from the effort and equally from the relief. It happens very often in life that when courage calls for self-control in the face of great matters, a happy or beneficial resolution will release a flood of tears of relief: once the need for great courage and strength is past, the nerves unravel and the depth of the fear or worry or pain roll to the surface. This is what the boy--very naturally and authentically--experienced: the relief that showed the depth of his struggle through unprovoked tears.
In the middle of the story,Schatz tried to overcome his emotional fear that he was going to die of fever .But towards the end of the story,when the misunderstanding of schatz was solved by his father,he was self-restrained.