Why is the ninth month called September?
late 14c., "a freedman, an emancipated slave," from Latin libertinus "member of a class of freedmen," from libertus "one's freedmen," from liber "free" (see liberal). Sense of "freethinker" is first recorded 1560s, from French libertin (1540s) originally the name given to certain Protestant sects in France and the Low Countries. Meaning "dissolute or licentious person" first recorded 1590s; the darkening of meaning being perhaps due to misunderstanding of Latin libertinus in Acts vi:9. As an adjective by 1570s.
found only Acts 6:9, one who once had been a slave, but who had been set at liberty, or the child of such a person. In this case the name probably denotes those descendants of Jews who had been carried captives to Rome as prisoners of war by Pompey and other Roman generals in the Syrian wars, and had afterwards been liberated. In A.D. 19 these manumitted Jews were banished from Rome. Many of them found their way to Jerusalem, and there established a synagogue.