The Unexpected Hanging

One night in Puzzlania a guilty man was brought before the judge for sentencing.

"Given the enormity of your crimes, I have no choice but sentence you to death by hanging. The execution shall be carried out at high noon, one day this coming week."

"Which day would that be?" the prisoner gulped.

"It is the decision of this court that the day of the hanging be a surprise to the prisoner; you shall not know for certain what day you will be hanged until one hour before the execution. This is the ruling of the court and so it shall stand."

As the prisoner was led back to his cell, to await a not particularly uplifting week, a thought occurred to him. It was the strict law in Puzzlania that judges must speak the truth at all times to the prisoners and accused. If a judge was found to be lying, he could be punished severely and the prisoner acquitted. That night, the prisoner wracked his brain and devised a legal defense based on the judge's decree.

What was the defense, and would it save him?