Puzzle time -the prisoner's 'theory'

Here is a puzzle...a judge awarded a prisoner death sentence, and the prisoner was to be hanged within a week. The judge gave him a concession that he (the prisoner) will not get to know the exact date on which he will be hanged, to keep his stress level under check.

Back in his room, this prisoner was trying to figure out when he could be hanged. First he looked at Saturday, the last day of the week. If he is to be hanged on Saturday, he will be alive on Friday. In other words, if he is alive on Friday, he will be definitely hanged on Saturday, which means he'll come to know his day of hanging. So Saturday cannot be his 'day'.

He looked at Friday next. If he is to be hanged on Friday, he will be alive on Thursday. If he is alive on Thursday, he will definitely be hanged on Friday, as Saturday is already ruled out. In other words, if he is alive on Thursday, he will come to know his day of hanging (will be Friday)! So, Friday can't be the 'day' either.

He applied this strategy to Thursday... and came up with the same answer -that he can't be hanged on Thursday either. The same approach was applicable to all days of the week - and our man 'figured out' that he could not be hanged on any day of the week!

What is wrong here? Or, is there anything wrong in the prisoner's 'theory'??