The Lion, the Unicorn, and Yesterday
In the Forest of Forgetfulness, the Lion lies every Monday, Tuesday, and Wednesday and tells the truth the other four days. The Unicorn lies every Thursday, Friday, and Saturday and tells the truth the other four days. One day you meet both, and each one tells you: 'Yesterday I was lying.' What day of the week is it?
Reveal the answer
It's Thursday. For 'yesterday I was lying' to be said truthfully today, or lied about today while true yesterday, today and yesterday must straddle that animal's lying/truth boundary — Wednesday/Thursday for the Lion, Saturday/Sunday for the Unicorn. Checking every day, only Thursday satisfies both statements at once. It's from Raymond Smullyan's 1978 collection, one of his many day-of-the-week spins on knights-and-knaves logic.
— Raymond Smullyan, What Is the Name of This Book? The Riddle of Dracula and Other Logical Puzzles — 1978
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