Valentine's Day is indeed a pagan holiday: https://www.history.com/topics/ancient-rome/lupercalia
Lupercalia was an ancient pagan festival held each year in Rome on "February 15. Although Valentine’s Day shares its name with a martyred Christian saint, some historians believe the holiday is actually an offshoot of Lupercalia. Unlike Valentine’s Day, however, Lupercalia was a bloody, violent and sexually-charged celebration awash with animal sacrifice, random matchmaking and coupling in the hopes of warding off evil spirits and infertility.
<Romulus and Remus>
No one knows the exact origin of Lupercalia, but it has been traced back as far as the 6th century B.C. According to Roman legend, the ancient King Amulius ordered Romulus and Remus—his twin nephews and founders of Rome—to be thrown into the Tiber River to drown in retribution for their mother’s broken vow of celibacy.
A servant took pity on them, however, and placed them inside a basket on the river instead. The river-god carried the basket and the brothers downriver to a wild fig tree where it became caught in the branches. The brothers were then rescued and cared for by a she-wolf in a den at the base of Palatine Hill where Rome was founded.
The twins were later adopted by a shepherd and his wife and learned their father’s trade. After killing the uncle who’d ordered their death, they found the cave den of the she-wolf who’d nurtured them and named it Lupercal.
It’s thought Lupercalia took place to honor the she-wolf and please the Roman fertility god Lupercus.
<Ritual Sacrifice>
Lupercalia rituals took place in a few places: Lupercal cave, on Palatine Hill and within the Roman open-air, public meeting place called the Comitium. The festival began at Lupercal cave with the sacrifice of one or more male goats—a representation of sexuality—and a dog. The sacrifices were performed by Luperci, a group of Roman priests. Afterwards, the foreheads of two naked Luperci were smeared with the animals’ blood using the bloody, sacrificial knife. The blood was then removed with a piece of milk-soaked wool as the Luperci laughed.
<Feast of Lupercal>
In Ancient Rome, feasting began after the ritual sacrifice. When the feast of Lupercal was over, the Luperci cut strips, also called thongs or februa, of goat hide from the newly-sacrificed goats. They then ran naked or nearly-naked around Palantine whipping any woman within striking distance with the thongs. Many women welcomed the lashes and even bared their skin to receive the fertility rite; it’s open to speculation what the lashes represented. During Lupercalia, the men randomly chose a woman’s name from a jar to be coupled with them for the duration of the festival. Often, the couple stayed together until the following year’s festival. Many fell in love and married. Over time, nakedness during Lupercalia lost popularity. The festival became more chaste, if still undignified, and women were whipped on their hands by fully-clothed men.
In Plutarch’s Life of Julius Caesar, Caesar famously refuses a golden crown presented to him by Mark Antony during the feast of Lupercalia.
<Saint Valentine>
There are several legends surrounding the life of Saint Valentine. The most common is that on one February 14 during the 3rd century A.D., a man named Valentine was executed by the Roman Emperor Claudius II after being imprisoned for assisting persecuted Christians and secretly marrying Christian couples in love. As the story goes, during Valentine’s imprisonment he tried converting Claudius to Christianity. Claudius became enraged and ordered Valentine to reject his faith or be killed. He refused to forsake his faith, so Valentine was beheaded. Legend also tells of another story that happened during Valentine’s imprisonment after he tutored a girl named Julia, the blind daughter of his jailer. The legend states God restored Julia’s sight after she and Valentine prayed together. On the eve of his execution, Valentine supposedly penned a note to Julia and signed it, “From your Valentine. Some historians believe more than one man named Valentine was executed by Claudius II. Despite the ambiguity surrounding Valentine and his life, the Catholic Church declared him a saint and listed him in Roman Martyrology as being martyred on February 14.