On Friday, the condemned criminal - let's call him Pete - was told by the Governor that he would be executed either on Saturday or Sunday. But whichever day was chosen, it would be a surprise.
Pete then reasoned as follows. If Sunday rolls around, then I shall know that I am to be executed that day and it will be no surprise. So they have to execute me on Saturday.
Ah, now I know that, it's clear that when they come for me on Saturday it will be no surprise either ... so that means they can't execute me on Saturday!
Pete went back to his cell that Friday afternoon happy in his absolute certainty that he would not be executed this coming weekend after all.
Imagine his surprise when they came for him next day!
Note: this well-known paradox indicates the slippery, multi-level nature of the concept of "surprise".