The Early Medieval acta of either Saint Valentine were expounded briefly in Legenda Aurea. According to that version, St Valentine was persecuted as a Christian and interrogated by Roman Emperor Claudius II in person. Claudius was impressed by Valentine and had a discussion with him, attempting to get him to convert to Roman paganism in order to save his life. Valentine refused and tried to convert Claudius to Christianity instead. Because of this, he was executed. Before his execution, he is reported to have performed a miracle by healing the blind daughter of his jailer.

Since Legenda Aurea still provided no connections whatsoever with sentimental love, appropriate lore has been embroidered in modern times to portray Valentine as a priest who refused an unattested law attributed to Roman Emperor Claudius II, allegedly ordering that young men remain single. The Emperor supposedly did this to grow his army, believing that married men did not make for good soldiers. The priest Valentine, however, secretly performed marriage ceremonies for young men. When Claudius found out about this, he had Valentine arrested and thrown in jail.

There is an additional modern embellishment to The Golden Legend, provided by American Greetings to History.com, and widely repeated despite having no historical basis whatsoever. On the evening before Valentine was to be executed, he would have written the first "valentine" card himself, addressed to the blind daughter of his jailor Asterius, signing as "From your Valentine.".

Totally stolen from Wikipedia.