Last Sunday, I read Saint Valentine by Robert Sabuda, to the kids during the Children's Sermon. I recommend it highly.
Valentine was martyred in the third century on February 14. That was one day before a traditional Roman holiday, Lupercalia, devoted to the remembrance of the she-wolf who, in ancient myth, suckled Romulus and Remus, the founders of Rome.
Saint Valentine was a real person though, and Christians' decision to celebrate his martyrdom around the same time as an already-established pagan holiday represents the ongoing genius of the Judeo-Christian faith for reclaiming and rebaptizing celebrations to honor the God revealed to ancient Israel and ultimately, in Jesus of Nazareth.
In Old Testament times, the ancient Israelites reclaimed the sites of altars to the numerous gods worshiped by others and devoted them to the worship of the one God of the universe. Christians would take a holiday like Saturnalia and reclaim it to celebrate the birth of Jesus, the Light of the world. In later centuries, Patrick would recast the traditions of the Irish people in order to teach and remind people of the truth of Jesus.
The book on Valentine is very good and I recommend it for use by parents, pastors, and Sunday School teachers who work with young people.