[danbruc]

Without having it thought out carefully I suggest the following. The judge's statement is impossible, i.e. it is not possible that the prisoner will be hanged on any day and is surprised by this. From this impossible statement - similar to a wrong premise - the prisoner draws the false conclusion that he will not be hanged at all and is then obviously surprised by someone knocking on his door.

[mcphage]

Well, I don't think you can conclude the judge's statement is impossible, since (at least in the story) it happened. So it had to be at least possible. (The judge doesn't claim that the hanging could happen any day and be a surprise, just that the day it happens it will be a surprise).