An ex-post-facto law is a law which imposes penalties retroactively, that is, upon acts already done, or which increases the penalty for the past acts.
 An ex post facto law (from the Latin for "from something done afterward")or retrospective law is a law that retrospectively changes the legal consequences of acts committed or the legal status of facts and relationships that existed prior to the enactment of the law. In reference to criminal law, it may criminalize actions that were legal when by bringing it into a more severe category than it was in at the time it was committed; or it may change or increase the punishment prescribed for a crime, such as by adding new penalties or extending terms; or it may alter the rules of evidence in order to make conviction for a crime more likely than it would have been at the time of the action for which a defendant is prosecuted. Conversely, a form of ex post facto law commonly known as an amnesty law may decriminalize certain acts or alleviate possible punishments (for example by replacing the death sentence with life-longimprisonment) retrospectively.
 A law may have an ex post facto effect without being technically ex post facto. For example, when a law repeals a previous law,The repealed legislation no longer applies to the situations it once did, even if such situations arose before the law was repealed. The principle of prohibiting the continued application of these kinds of laws is also known as Nullum crimen,nulla poena sine praevia lege poenali.
 Generally speaking, ex post facto laws are seen as a violation of the rule of law as it applies in a free and democratic society. Most common law jurisdiction do not permit retrospective legislation, though some have suggested that judge-made law is retrospective as a new precedent applies to events that occurred prior to the judicial decision. In some nations that follow the Westminster system of government,such as the United Kingdom, ex post facto laws are technically possible as the doctrine of parliamentary supremacy allows parliament to pass any law it wishes. However, in a nation with an entrenched bill of rights or a written constitution, ex post facto legislation may be prohibited.
Ex-post facto Laws
Latin for "after the fact," which refers to laws adopted after an act is committed making it illegal although it was legal when done, or increasing the penalty for a crime after it is committed. Such laws are specifically prohibited by the U.S. Constitution. Therefore, if a state legislature enacts new rules of proof or longer sentences, those new rules or sentences do not apply to crimes committed before the new law was adopted.
In the field of criminal law an ex post facto legislation, is prohibited. We have therefore to find out the scope of such legislation and how far the present Ordinance is a retroactive law. In the matter of ex post facto legislation having retroactive effect, there is considerable difference between what obtains in the United States of America and what is in vogue in England. At page 383 of "Law in the Making" by C. K. Allen, the learned author observes as follows:
"There is, in all civilized states, the strongest prejudice against retrospective or ex post facto legislation, since it is, in the words of Willes J., 'prima facie' of questionable policy, and contrary to the general principle that...