Valentine's Day, also called Saint Valentine's Day or the Feast of Saint Valentine, is celebrated annually on February 14. It originated as a Christian feast day honoring a martyr named Valentine, and through later folk traditions it has also become a significant cultural, religious and commercial celebration of romance and love in many regions of the world.

There are a number of martyrdom stories associated with various Saint Valentines connected to February 14, including an account of the imprisonment of Saint Valentine of Rome for ministering to Christians persecuted under the Roman Empire in the third century. According to an early tradition, Saint Valentine restored sight to the blind daughter of his jailer. Numerous later additions to the legend have better related it to the theme of love: tradition maintains that Saint Valentine performed weddings for Christian soldiers who were forbidden to marry by the Roman emperor; an 18th-century embellishment to the legend claims he wrote the jailer's daughter a letter signed "Your Valentine" as a farewell before his execution.

The 8th-century Gelasian Sacramentary recorded the celebration of the Feast of Saint Valentine on February 14. The day became associated with romantic love in the 14th and 15th centuries, when notions of courtly love flourished, apparently by association with the "lovebirds" of early spring. In 18th-century England, it grew into an occasion for couples to express their love for each other by presenting flowers, offering confectionery, and sending greeting cards (known as "valentines"). Valentine's Day symbols that are used today include the heart-shaped outline, doves, and the figure of the winged Cupid. In the 19th century, handmade cards gave way to mass-produced greetings. In Italy, Saint Valentine's keys are given to lovers "as a romantic symbol and an invitation to unlock the giver's heart", as well as to children to ward off epilepsy (called Saint Valentine's Malady).

It is a day of commemoration in the Anglican Communion and the Lutheran Church. Many parts of the Eastern Orthodox Church celebrate Saint Valentine's Day on July 6 in honor of Roman presbyter Saint Valentine, and on July 30 in honor of Hieromartyr Valentine, the Bishop of Interamna (modern Terni).

Numerous early Christian martyrs were named Valentine. The Valentines honored on February 14 are Valentine of Rome (Valentinus presb. m. Romae) and Valentine of Terni (Valentinus ep. Interamnensis m. Romae). Valentine of Rome was a priest in Rome who was martyred in 269 and was buried on the Via Flaminia. The relics of Saint Valentine were kept in the Church and Catacombs of San Valentino in Rome, which "remained an important pilgrim site throughout the Middle Ages until the relics of Saint Valentine were transferred to the church of Santa Prassede during the pontificate of Nicholas IV [1288–1292]". The flower-crowned skull of Saint Valentine is exhibited in the Basilica of Santa Maria in Cosmedin, Rome. Other relics are found at Whitefriar Street Carmelite Church in Dublin, Ireland.

Valentine of Terni became bishop of Interamna (now Terni, in central Italy) and is said to have been martyred during the persecution under Emperor Aurelian in 273. He is buried on the Via Flaminia, but in a different location from Valentine of Rome. His relics are at the Basilica of Saint Valentine in Terni (Basilica di San Valentino). Professor Jack B. Oruch of the University of Kansas notes that "abstracts of the acts of the two saints were in nearly every church and monastery of Europe." A relic claimed to be Saint Valentine of Terni's head was preserved in the abbey of New Minster, Winchester, and venerated.

The Catholic Encyclopedia speaks of a third saint named Valentine who was mentioned in early martyrologies under date of February 14. He was martyred in Africa with a number of companions, but nothing more is known about him.

February 14 is celebrated as Saint Valentine's Day in various Christian denominations; it has, for example, the rank of "commemoration" in the calendar of saints in the Anglican Communion. The feast day of Saint Valentine is given in the calendar of saints of the Lutheran Church. In the 1969 revision of the Roman Catholic Calendar of Saints, the feast day of Saint Valentine on February 14 was relegated from the General Roman Calendar to particular (local or even national) calendars for the following reason: "Though the memorial of Saint Valentine is ancient, it is left to particular calendars, since, apart from his name, nothing is known of Saint Valentine except that he was buried on the Via Flaminia on February 14." Therefore, as he remains within the Roman Martyrology, he may be recognised optionally during mass outside of Christmastide and Eastertide.