Not much apparently.....
There were a few Saints named Valentine, two of whom have February 14th as their designated day. As with most Saints, stories of their martyrdoms are embellished and often contradictory.
Although taking place about the time of the persecutions by Aurelius, the first literary reference didn't appear until the 14th century, even then the link is tenuous. Chaucer wrote in 'Parlement of Foules' (1382) to honour the first anniversary of the engagement of Richard II to Anne of Bohemia
'for this was on seynt volantynys day,whan euery bryd comyth there to chese his make' (......there to choose his mate)
It was assumed Chaucer was speaking of one of the February Valentines but the treaty of marriage was signed on May 2nd 1381, they married eight months later both aged fifteen. That would make Chaucer's Valentine (the first ever to be associated with love), Valentine of Genoa whose day is May 3rd and died around 307 AD.
So there it is, not only is everybody associating this love thing with the wrong Valentine, it was never about him anyway.
Happy Valentine's Day!