Well, it is the feast of an early Church martyr.
Roman Martyrology commemorates the martyrdoms of two individuals, both named Valentine, that took place on February 14. Both were beheaded, one at Rome and the other at Terni (sixty miles away). There is some debate whether there was only one Valentine who was the bishop of Terni, but brought to Rome to be martyred or if there was a second Valentine who, according to some ancient sources, was a priest in Rome who was martyred during the persecutions of Claudius the Goth. If there was only one, it is possible that St. Valentine was honored in both Rome and Terni, later leading historians to conclude they were two different people.
Regardless, it was not until the latter part of the Middle Ages that the custom of sending love letters on St. Valentine’s Day came about. Commercialism and secularization has left St. Valentine largely forgotten by those who invoke his name: “Be mine, Valentine.”
Further, it’s the day the Church celebrates the feast of Cyril and Methodius, “Apostles to the Slavs,” about whom Blessed John Paul II wrote an encyclical, Slavorum Apostoli.