Article I, Section 10 of our Constitution declares that “no…ex post facto law shall be passed.” Literally, the phrase means “after the fact”; the doctrine prohibiting the passage of ex post facto laws generally applies to criminal laws which, after an act has been committed, make a previously legal act a crime, or those laws which increase the penalties for criminal conduct after the conduct has taken place.
Stated differently, the Constitution insures that the government cannot change the rules of the game after the game has begun. The doctrine opposing ex post facto laws is one of fundamental fairness and is a hallmark of our free society.
— BBC&B