This is a classic paradox that goes something like this:
A man is to be sentenced to death, but due to the severity of his crime, the judge decrees that the man is to be hanged one morning next week and he is not to know the exact date of his hanging.
The man is safe in the knowledge that he cannot be hanged. Why? Well it is a proof by induction. Suppose Friday is the last day of the week. If the man is not hanged by Thursday then he will know he is going to be hanged on Friday. Therefore he cannot be hanged on Friday. What about Thursday? Well if the man hasn't been hanged by Wednesday then he knows he is going to be hung on Tursday (since Friday has already been ruled out by the above logic). This rules out Wednesday as well. Carrying on this logic you can rule out all the days as well.