While the Church recognizes 14 saintly martyrs named Valentine, the earliest known one was St. Valentinus, a 3rd century Catholic who refused to obey Emperor Claudius The Cruel's dictate to worship 12 gods. Valentinus was arrested and imprisoned. His jailer surreptitiously asked Valentinus to teach his blind daughter, Julia. Valentinus taught her the beauty of God's creation, as well as how to pray. Julia believed and humbly prayed for healing, which God granted in a flash of bright light. On the eve of Valentinus's execution (February 14, 270 A.D.), he wrote Julia a note encouraging her always to stay close to God, and signed it, "From Your Valentine."
And that, in the immortal words of Paul Harvey, is the rest of the story!