The Roman calendar, like so much of the liturgy, was reformed following the Second Vatican Council. The reason for St. Valentine not having a memorial on the reformed calendar is that St. Valentine's existence, like that of St. Christopher and a few other "saints," cannot be pinned down with any certainty. It is likely that St. Valentine, as he came to be remembered and memorialized, was a composite figure. He is listed as a Roman priest/martyr and, as such, appears in some Roman martryologies, though none that date back to the times of Christian persecution, but only in some dating back no later than the fifth century. The name Valentinus is also associated as a North African martyr, as well as the bishop of ancient Interamn, which is modern Terni. All of this information is readily available in the on-line Catholic Encyclopedia.
One of the reasons for reforming the Roman liturgical calendar was to eliminate the feasts of saints whose actual existence was dubious. Christianity is, after all, an historical religion and saints are real people, this is no small thing.
The same Encyclopedia article goes on to state that "popular customs associated with Saint Valentine's Day undoubtedly had their origin in a conventional belief generally received in England and France during the Middle Ages, that on 14 February, i.e. half way through the second month of the year, the birds began to pair." As a demonstration of this idea, many historians point to lines 309, 322 386 of Sir Geoffrey Chaucer's poem, Parliament of Foules, which the poet later re-titled Book of St. Valentine's Day of the Parliament of Birds:
309 For this was on seynt Valentynes day,Of course, this should do nothing to dampen having a romantic supper and evening with your beloved. Enjoy! Clarifying this, in my estimation, does help to insulate faith from a kind pie-eyed, squishy sentimentalism and remind us that hedonism is never alright. In any case, true love is a kind of acesis.
310 Whan every foul cometh ther to chese his make,
311 Of every kinde, that men thenke may;
312 And that so huge a noyse gan they make,
313 That erthe and see, and tree, and every lake
314 So ful was, that unnethe was ther space
315 For me to stonde, so ful was al the place