The Catholic Church mentions at least three different Saint Valentines, all of them martyrs, in the early martyrologies, under the date of February 14th.
One is described as a priest at Rome, another as bishop of Interamna (modern day Terni). These two both suffered in the second half of the third century, and were buried on the Flaminian Way, but at different distances from the city. What was known to the ancients as the Flaminian Gate of Rome, and is now the Porta del Popolo, was once called the Gate of St. Valentine. The name seems to have been taken from a small church dedicated to the saint, which was in the immediate neighborhood.
Of both these St. Valentines, some sort of Acta (records of important acts) are preserved, but they are of relatively late date and of no historical value. Of the third Saint Valentine, who suffered in Africa with a number of companions, there is no further information.
The above is probably all we know for sure, but the legends offer more detail..
During Valentine's priesthood, the Roman Emperor, Claudius II, started persecuting and imprisoning Christians for not worshiping the Roman gods. During this period, war was raging.
Claudius began calling for all the able-bodied men to go into battle with him, but many of the Romans were unwilling to sign up. The married men did not want to leave their families, and the younger men did not wish to leave their sweethearts. Ever the politician, Claudius figured that he could abolish love by decree. He ordered that no marriages should be celebrated and that all engagements must be broken off immediately.
Valentine didn't buy into this, and started to perform secret marriages, soon gaining a reputation as a friend of lovers and sweethearts in every district of Rome. He was also helping Christians escape the clutches of the Emperor. Not surprisingly, Valentine ended up in prison.
To be a saint, though, a miracle is needed. Not to worry...
While in prison, Valentine continued to minister to anyone he could. One of the guards, a man named Asterius, had adopted a blind girl many years earlier, when she was just a little child. He asked Valentine if his God could help his daughter. Valentine prayed for her, and her sight was restored. Because of that, the guard and his whole family -- all his relatives that were living in that area -- 46 people -- believed and were baptized.
Soon after, Valentine was tortured and beaten with clubs, and then was beheaded. The date was Feb. 14th, in the year 270 A.D.
Valentine fell in love with the jailer's daughter that had been healed. Just before he was led out to his execution, he managed to write one last note to her, a farewell message, and he signed it "From Your Valentine."
The custom of sending lover's greetings on February 14th has no direct connection with either Valentine, but dates from the later Middle Ages, when it was believed that this day marked the beginning of the mating season for birds.