Ex post facto
Ex post facto is Latin for "after the fact", or, retroactively.
In law
It most often comes up in the context of a prohibition on ex post facto laws or enforcement of them — that is, if the law governing an activity is passed after the activity occurs, one cannot be retroactively punished.
The most authoritative statement of this can be found in Article 11(2) of The Universal Declaration of Human Rights — "No one shall be held guilty of any penal offence on account of any act or omission which did not constitute a penal offence, under national or international law, at the time when it was committed. Nor shall a heavier penalty be imposed than the one that was applicable at the time the penal offence was committed."
So if you've done a massive amount of masturbating on Monday, and they make it illegal on Tuesday, you won't get done for it. And if they introduce the death penalty for masturbating on the Wednesday, they can't kill you for it either. The lesson here, kids, is to make sure you keep up to date with the legislative programme of the legislature(s) that has jurisdiction over you and plan your self-love sessions accordingly.
Predictions
Ex post facto predictions (also known as vaticinium ex eventu, prophecy [derived from] the occurrence) are ones that are not made until the facts are available — i.e., explanations of events that sound "predictive" but are not made in advance of specific occurrences, but after them. Similarly, claiming given events fulfill prophecy after they occur allows wide leeway in interpretation, as opposed to interpreting the prophecy before the events and accurately predicting how they will occur.
It's easy to trick people into believing you made a true prediction, when in reality it was post facto. This can be done through clever misdirection and is often a trick used by mediums or other mind-readers. Cold readers can be quite good at getting people to reveal facts and then claiming that these facts were divined through psychic powers and were actually predicted. However, it doesn't always work. In 2009, mentalist Derren Brown attempted such a feat by "predicting" the results of the National Lottery draw. Using the same misdirection methods he uses in mental illusions and card tricks, he tried to convince the audience that he's predicted the results beforehand. However, most people seemed to see past the tomfoolery and clearly noticed that he revealed his "prediction" a few seconds after the result was shown — with the illusion being due to a camera trick.