It's what the Supreme Court defined the ex post facto clauses of the Constitution to mean in Calder v. Bull in 1798. Justice Chase (no, not that one) writing for the majority said:
"I shall endeavour to show what law is to be considered an ex post facto law, within the words and meaning of the prohibition in the Federal Constitution. The prohibition, "that no state shall pass any ex post facto law," necessarily requires some explanation; for, naked and without explanation, it is unintelligible, and means nothing. Literally, it is only, that a law shall not be passed concerning, and after the fact, or thing done, or action committed. I would ask, what fact; of what nature, or kind; and by whom done? That Charles 1st. king of England, was beheaded; that Oliver Cromwell was Protector of England; that Louis 16th, late King of France, was guillotined; are all facts, that have happened; but it would be nonsense to suppose, that the States were prohibited from making any law after either of these events, and with reference thereto. The prohibition, in the letter, is not to pass any law concerning, and after the fact; but the plain and obvious meaning and intention of the prohibition is this; that the Legislatures of the several states, shall not pass laws, after a fact done by a subject, or citizen, which shall have relation to such fact, and shall punish him for having done it. The prohibition considered in this light, is an additional bulwark in favour of the personal security of the subject, to protect his person from punishment by legislative acts, having a retrospective operation."
To cite that case is a straw man, non-seq, as it says nothing of the circumstances I described.
Here is what the Supreme Court has said on that subject though:
"An ex post facto law is one which renders an act punishable in a manner in which it was not punishable when it was committed. Such a law may inflict penalties on the person, or may inflict pecuniary penalties which swell the public treasury." - John Marshall, Fletcher v. Peck, 1810
In other words, ex post facto laws work both ways, their only criterion being that they alter the manner in which a given act is punishable from the time it was committed.
Story's Commentaries echo this sentiment:
"§ 1339. Of the same class are ex post facto laws, that is to say, (in a literal sense,) laws passed after the act done. The terms, ex post facto laws, in a comprehensive sense, embrace all retrospective laws, or laws governing, or controlling past transactions"
Also, check in your own case of Calder v. Bull. You will find the following, which specifically mentions a certain type of Ex Post Facto law that lessens a punishment for an offense:
"All the restrictions contained in the constitution of the United States on the power of the state legislatures, were provided in favor of the authority of the federal government. The prohibition against their making any ex post facto laws was introduced for greater caution, and very probably arose from the knowledge, that the parliament of Great Britain claimed and exercised a power to pass such laws, under the denomination of bills of attainder, or bills of pains and penalties; the first inflicting capital, and the other less punishment."