Valentine's Day surged in popularity in the Middle Ages because of the influence of courtly love and romantic literature.
During the Middle Ages, courtly love was a common practice among the upper classes of Europe. It was a concept that celebrated the ideal of love, which often involved unattainable or illicit love affairs. The idea of courtly love was popularized through literature, such as the Arthurian romances, and it often featured the exchange of love tokens, such as flowers or poems.
One of the most famous stories associated with Valentine's Day is the tale of Saint Valentine, a Christian martyr who was executed in Rome in the 3rd century for performing weddings for soldiers, who were forbidden to marry, and for ministering to Christians. According to legend, he sent a letter to his jailer's daughter signed "from your Valentine" before he was executed. This story and others like it were later popularized in literature and contributed to the romanticization of the holiday.
Over time, Valentine's Day became associated with the exchange of love notes, flowers, and other tokens of affection, and it evolved into the holiday that is celebrated today. By the 15th century, it had become a popular tradition among the upper classes in Europe, and it eventually spread throughout the world.