VALENTINE was a holy priest in Rome, who, with St.  Marius and his family, assisted the martyrs in the persecution under Claudius II. He was apprehended, and sent by the emperor to the prefect of Rome, who, on finding all his promises to make him renounce his faith ineffectual, commanded him to be beaten with clubs, and afterward to be beheaded, which was executed on the 14th of February, about the year 270. Pope Julius I, is said to have built a church near Ponte Mole to his memory, which for a long time gave name to the gate now called Porta del Popolo, formerly Porta Valentini. The greater part of his relics are now in the Church of St. Praxedes. To abolish the heathens’ lewd superstitious custom of boys ‘ drawing the names of girls, in honor of their goddess Februata Juno, on the 15th of this month, several zealous pastors substituted the names of Saints in billets given on this day.

(Source: Butler’s Lives of the Saints)

At least three different Saint Valentines, all of them martyrs, are mentioned in the early martyrologies under date  of 14 February. One is described as a priest at Rome, another as bishop of Interamna (modern Terni), and these two seem both to have suffered in the second half of the third century and to have been buried on the Flaminian Way, but at different distances from the city. In William of Malmesbury’s time what was known to the ancients as the Flaminian Gate of Rome and is now the Porta del Popolo, was 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 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, nothing further is known.

Source: Wikipedia