The Day that (N)ever comes
There is a jokey story about a judge condemning to death a murderer whose crime was so horrifying that the judge decided to add to his punishment by withholding the date of his execution. He said, “You will be executed some time next week, Monday to Sunday inclusive, and it will be on a day you do not expect.”
In his cell the murderer began his calculation: “If I reach Saturday night, it means I will have to be executed on the following Sunday. I would know; I would expect a Sunday execution. But the judge it would be on a day I would not expect, so I can’t possibly be executed on Sunday. I can rule out Sunday completely. So it must be one of the days Monday to Saturday.”
But then he thought: “ I cannot be executed on the Saturday! If I reach Friday night I cannot be executed on Saturday, because I would expect it! So I cannot be executed on Sunday or Saturday. But that same reasoning applies to all the days of the week! I cannot be executed at all!”
To his great horror they turned up on Wednesday and he was executed.
This puzzle arises because of a collision between something indefinite (the “unexpected”) and something definite (definite days of the week). It’s a well-known logical conundrum.