Numerous early Christian martyrs were named Valentines. The Valentines honored on February 14 are Valentine of Rome and Valentine of Terni. Valentine of Rome was a priest in Rome who was martyred in 269 and was added to the calendar of saints by Pope Gelasius I in 496, 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 St. Valentine were transferred to the church of Santa Prassede during the pontificate of Nicholas IV." 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. 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 in Europe. The Catholic Encyclopedia also speaks of a third saint named Valentine, mentioned in early martyrologies under February 14. He was martyred in Africa with several companions, but nothing more is known about him. A relic claimed to be Saint Valentine of Terni's head was preserved in the abbey of New Minster, Winchester, and venerated.

February 14 is celebrated as St. Valentine's Day in various Christian denominations; it has, for example, the rank of 'commemoration' in the calendar of saints in the Anglican Communion. In addition, the feast day of Saint Valentine is also given in the calendar of saints of the Lutheran Church. However, in the 1969 revision of the Roman Catholic Calendar of Saints, the feast day of Saint Valentine on February 14 was removed from the General Roman Calendar and relegated to particular 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.

The feast day is still celebrated in Malta, where relics of the saint are claimed to be found, and also throughout the world by Traditionalist Catholics who follow the older, pre-Second Vatican Council calendar.

In the Eastern Orthodox Church, St. Valentine is recognized on July 6, in which Saint Valentine, the Roman presbyter, is honored; in addition, the Eastern Orthodox Church observes the feast of Hieromartyr Valentine, Bishop of Interamna, on July 30.

The "Feast" on the birthday) Saint Valentine originated in Christendom and has been marked by the Western Church of Christendom in honor of one of the Christian martyrs named Valentine, as recorded in the 8th-century Gelasian Sacramentary. In Ancient Rome, Lupercalia was observed February 13–15 on behalf of Pan & Juno, pagan gods of love, marriage & fertility. It was a rite connected to purification and health, with only a slight connection to fertility (as a part of health) and none to love. The celebration of Saint Valentine is not known to have had any romantic connotations until Chaucer's poetry about "Valentine's Day" in the 14th century, some seven hundred years after the celebration of Lupercalia is believed to have ceased. Lupercalia was a festival local to the city of Rome. The more general Festival of Juno Februa, meaning "Juno the purifier" or "the chaste Juno," was celebrated on February 13–14. Although Pope Gelasius I's (492–496) article in the Catholic Encyclopedia says that he abolished Lupercalia, Sioux City UMC Professor Bruce Forbes wrote that "no evidence" has been demonstrated to link St. Valentine's Day and the rites of the ancient Roman purification festival of Lupercalia, despite claims by many authors to the contrary.

Some researchers have theorized that Gelasius I replaced Lupercalia with the celebration of the Purification of the Blessed Virgin Mary and claim a connection to the 14th century's connotations of romantic love. Still, there is no historical indication that he intended such a thing. Also, the dates do not fit because, at the time of Gelasius I, the feast was only celebrated in Jerusalem, and it was on February 14 only because Jerusalem placed the Nativity of Jesus (Christmas) on January 6. Although it was called "Purification of the Blessed Virgin Mary," it also dealt with the presentation of Jesus at the temple. Jerusalem's Purification of the Blessed Virgin Mary on February 14th became the Presentation of Jesus at the Temple on February 2 as it was introduced to Rome and other places in the sixth century, after Gelasius I's time.

Alban Butler, in his The Lives of the Fathers, Martyrs, and Other Principal Saints (1756–1759), claimed without proof that men and women in Lupercalia drew names from a jar to make couples. Modern Valentine's letters originated from this custom. In reality, this practice originated in the Middle Ages, with no link to Lupercalia, with men drawing the names of girls at random to couple with them. This custom was combated by priests, for example, by Frances de Sales around 1600, apparently by replacing it with a religious custom of girls drawing the names of apostles from the altar. However, this religious custom is recorded as soon as the 13th century in the life of Saint Elizabeth of Hungary so it could have a different origin.

Geoffrey Chaucer by Thomas Hoccleve (1412)

The first recorded association of Valentine's Day with romantic love is believed to be in the Parliament of Fowls (1382) by Geoffrey Chaucer, a dream vision portraying a parliament for birds to choose their mates. Honoring the first anniversary of the engagement of fifteen-year-old King Richard II of England to fifteen-year-old Anne of Bohemia, Chaucer wrote:

In modern English:


"For this was on Saint Valentine's Day
When every bird comes there to choose his match
Of every kind that men may think of
And that so huge a noise they began to make
That earth and air and tree and every lake
Was so full, that not easily was there space
For me to stand—so full was all the place."

The earliest description of February 14 as an annual celebration of love appears in the Charter of the Court of Love. The charter, allegedly issued by Charles VI of France at Mantes-la-Jolie in 1400, describes lavish festivities to be attended by several members of the royal court, including a feast, amorous song, and poetry competitions, jousting, and dancing. Amid these festivities, the attending ladies would hear and rule on disputes from lovers. No other court record exists, and none of those named in the charter were present at Mantes except Charles's queen, Isabeau of Bavaria, who may well have imagined it all while waiting out a plague.

In 1797, a British publisher issued The Young Man's Valentine Writer, which contained scores of suggested sentimental verses for the young lover unable to compose his own. Printers had already begun producing a limited number of cards with verses and sketches called "mechanical valentines." Paper Valentines became so popular in England in the early 19th century that they were assembled in factories. Fancy Valentines were made with real lace and ribbons, with paper lace introduced in the mid-19th century. In 1835, 60,000 Valentine's cards were sent by post in the United Kingdom, despite expensive postage.

A reduction in postal rates following Sir Rowland Hill's postal reforms with the 1840 invention of the postage stamp (Penny Black) saw the number of Valentines posted an increase, with 400,000 sent just one year after its invention, and ushered in the less personal but easier practice of mailing Valentines. That made it possible for the first time to exchange cards anonymously, which is taken as the reason for the sudden appearance of racy verse in an era otherwise prudishly Victorian. Production increased, "Cupid's Manufactory," as Charles Dickens termed it, with over 3,000 women employed in manufacturing. The Laura Seddon, Greeting Card Collection at Manchester Metropolitan University, gathers 450 Valentine's Day cards from early nineteenth-century Britain, printed by the major publishers of the day. The collection appears in Seddon's book Victorian Valentines (1996).

In the United States, the first mass-produced Valentines of embossed paper lace were produced and sold shortly after 1847 by Esther Howland (1828–1904) of Worcester, Massachusetts. Her father operated a large book and stationery store, but Howland took her inspiration from an English Valentine she had received from a business associate of her father. Intrigued with making similar Valentines, Howland began her business by importing paper lace and floral decorations from England. A writer in Graham's American Monthly observed in 1849, "Saint Valentine's Day ... is becoming, nay it has become, a national holiday." The English practice of sending Valentine's cards was established enough to feature as a plot device in Elizabeth Gaskell's Mr. Harrison's Confessions (1851): "I burst in with my explanations: 'The valentine I know nothing about.' 'It is in your handwriting, said he coldly." Since 2001, the Greeting Card Association has been giving an annual "Esther Howland Award for a Greeting Card Visionary."

Since the 19th century, handwritten notes have given way to mass-produced greeting cards. In the UK, just under half of the population spends money on their Valentines, and around £1.9 billion was spent in 2015 on cards, flowers, chocolates, and other gifts. The mid-19th century Valentine's Day trade was a harbinger of further commercialized holidays in the U.S.

A gift box of chocolates, which is a common gift for Valentine's Day

In 1868, the British chocolate company Cadbury created Fancy Boxes – a decorated box of chocolates – in the shape of a heart for Valentine's Day. Boxes of filled chocolates quickly became associated with the holiday. In the second half of the 20th century, exchanging cards was extended to all gifts, such as giving jewelry.

The U.S. Greeting Card Association estimates that approximately 190 million valentines are sent annually in the US. Half of those valentines are given to family members other than husband or wife, usually to children. When the valentine-exchange cards made in school activities are included, the figure goes up to 1 billion, and teachers become the people receiving the most valentines. The average valentine's spending has increased every year in the U.S, from $108 a person in 2010 to $131 in 2013.

The rise of Internet popularity at the turn of the millennium is creating new traditions. Millions of people use every year, digital means of creating and sending Valentine's Day greeting messages, such as e-cards, love coupons, or printable greeting cards. Some consider Valentine's Day a Hallmark holiday due to its commercialization.

In the modern era, liturgically, the Lutheran Church and Anglican Church have a service for St. Valentine's Day (the Feast of St. Valentine), which includes the optional rite of the renewal of marriage vows. In 2016, the Catholic Bishops of England and Wales established a novena prayer "to support single people seeking a spouse ahead of St Valentine's Day."