I feel like the prisoner's explanation is circular logic—akin to reducing something down to 0 = 0:

1. He claims he can't be executed on Friday, because it wouldn't be a surprise.

2. He then argues similarly that he can't be executed any other day.

3. He concludes therefore that he can't be executed.

But if "I won't be executed at all" was a possibility, then he could never even rule out Friday: on Friday, he'll either get executed, or not at all. Both are possible, so he can't claim an execution on Friday would be predictable beforehand.