According to wiki, the day was first celebrated by the giving of gifts to symbolise love in the C18. But Saint Valentine's day was recognised in Chaucer's day.
The date is that on which he was executed in the reign of the Roman Emperor, Claudius Gothicus 213- 217 AD. He was made a saint some time in the Middle Ages; until 993, saints were never formally created but owed their sanctity to tradition, but the year of his being formally beatified is not known, if he ever was.
It may be that February 14th was taken by early Christians as a convenient time to replace the Roman festival of Lupercalia. According to legend, Saint Valentine was executed for conducting Christian marriages of Roman soldiers. That legend seems to be his only connection with love or marriage.