Today, Valentine's Day looks like an innocent celebration of love, but it originated from an ancient pagan fertility ritual, which was then later "Christianized" by the Roman Catholic Church. The mere fact that this is a pagan holiday means that it is forbidden by Deuteronomy 12. For God commanded us not to mimic the ways of the pagans. Also, since the Catholic Church is pagan, anyone who tries to assert that this day isn't pagan because it was created by the Catholic Church is actually proving the day to be pagan.
This fertility ritual would start with the sacrifice of a goat and a dog. Then they would make strips of goat skin, drip them in blood, and slap their crops and young women with those strips. And the women, believing that this would make them more fertile, would line up to slapped with these strips of goat skin. Now, since this was a pagan fertility ritual, it was of course done in worship of a pagan god. This "god" is Cupid, who is depicted as a naked baby with angel-like wings, and equipped with a bow and arrow. Whoever got shot by this arrow would fall in love with someone. Modern depictions of this false god have it covered with some sort of cloth for "modesty". Anyone claiming to be Christian that celebrates Valentine's Day worships Cupid, not Yeshua.
Of course, this isn't the only thing that was done in the middle of the second month. I like to refer to Valentine's Day as "adultery feast". Why? Because as part of this pagan fertility ritual, the young women would put their names inside a large urn, and the young men would take a card out of the urn, and whoever's name was on that card got to be his partner in sex for the year. Sometimes, this would end in marriage. The "adultery" refers to what would happen if it didn't result in marriage. Though, technically speaking, adultery is when a man has sexual relations with another man's wife, not with an unmarried woman, as would be the case here.
By the way, have you noticed that this ancient ritual is a perversion of something that Yahweh called "very good"? Yahweh created mankind to be fertile, along with everything required to produce offspring. He created man to love women and women to love men. And he calls this "very good" (Genesis 1:31). The pagans somehow got the idea that there needed to be a ritual involving animal sacrifices to a god in order to increase women's fertility. Fertility that is already there by design! They also pervert that which Yahweh intended to lead to marriage into something sexually impure; into a ritual that results in a type of relationship that Yahweh requires to end in marriage. And finally, they turned an attribute of Yahweh into a god who's very character is hostile to the ways of Yahweh!