OT:
morolf: I'm not sure that's correct. It's true however that 25th December was the birthday of Sol invictus, the Sun god, and that probably did play a role for setting Christmas on that date.
At this time there were holidays all over the northern hemisphere celebration mid-winter, in Europe mostly celebrating the birth of some sun god or spirit. In Rome the most prominent by far was Mithras (inspired by the persian god Mitra) who really had a lot of similarity with Christ.
The strong Mithraism is probably the main reason Pope Julius I declared Dec 25th also as Christ's birthday. This however became only official for all important (then) Christian sects in 787 when they managed to agree with the decrees of the 2nd Council of Nicaea (which by the way also decreed the altar and crucifix as holy things and the worship of Mary as officially canon - among other stuff).
The decrees made by this council weren't openly questioned until the 16th century when John Calvin came along.
/OT
Half-OT
Valentine's day goes back to the Roman festival of Lupercalia (which was on Feb 15th). The origins of that one are lost in time, but it probably derives from a mix of Etruscan traditions, Greek celebrations surrounding Pan and the founding myths of Rome (Wolf-feast, from Romulus and Remus feeding off a she-wolf's milk).
Like with many holidays it was made an official Christian holiday (in this case in 496) because people just wouldn't stop celebrating it.
For that reason they invented the christian martyr Valentinus who supposed was executed on Feb 14th 273 (although that wasn't on the "official list of martyrs" in 354).
The connection to "a lover's day" was made in the 14th century by Chaucer and the people surrounding him. Or maybe re-made, because a lot of Chaucer's work references old Pagan myths and traditions and the Lupercalia and related holidays probably (like many Pagan holidays) were a lot about "getting it on".
/Half-OT