The United States Constitution prohibits the use of ex post facto laws to create situations where someone is punished for an offense that was committed in the past when their action was not considered to be punishable. This prohibition on creating laws that can be applied to past situations is important in ensuring our freedom. However, ex post facto remedies are sometimes allowed in civil rights rulings.
Clause 3 of Article I, Section 9 of the United States Constitution prohibits Congress from passing ex post facto laws. Clause 1 of Article I, Section 10 of the U.S. Constitution disallows states from passing such laws. When ruling in ex post facto law over time, the case that the Supreme Court turns to for guidance is its ruling in Calder v. Bull 3 U.S. 386 (1798). It was in that ruling that Justice Samuel Chase noted that the prohibition on ex post facto laws did not pertain to civil matters, but rather, only to criminal ones.
If you were the victim of discrimination that occurred prior to the passing of a specific law connected to the discriminatory action, you may still be granted legal remedies by the court in an ex post facto ruling, but retroactive remedies are often not favored by the courts even in civil lawsuits. Thus, if you are involved in a civil rights case as the plaintiff, it may or may not be a possible remedy.