[p1esk]

Here's that line again: "He will not know the day of the hanging until the executioner knocks on his cell door at noon that day".

The judge not only said that the prisoner will be surprised, he also said precisely when the prisoner will be surprised, which is noon on Friday, for Friday execution.

It does not matter if he gets surprised by the absence of the knock on Thursday. All that matters is that he would not be surprised when the knock happens on Friday.