In short, it was a religious pagan holiday day. Then it was Christianized and adopted by the Christians. So celebrating religious holidays is of other faiths is haram.
It was also a pagan ritual which pagans of today who may follow the old ways may or may not still engage in it. It is haram to adopt pagan/kuffar rituals.
It is something the kuffars engage in that promotes pre-marital relationships. Imitation of the non-believers is haram, as is pre-marital relationships and anything that leads to such relationships.
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The first information about this day is found in pre-Christian Rome, when pagans would celebrate the "Feast of the Wolf" on February 15, also known as the Feast of Lupercalius in honor of Februata Juno, the Roman goddess of women and marriage, and Pan, Roman god of nature.
On this day, young women would place their names in an urn, from which boys would randomly draw to discover their sexual companion for the day, the year, and sometimes the rest of their lives. These partners exchanged gifts as a sign of affection, and often married.
When Christianity came onto the scene in Rome, it wanted to replace this feast with something more in line with its ethics and morality.
A number of Christians decided to use February 14 for this purpose. This was when the Italian Bishop Valentine was executed by the Roman Emperor Claudius II for conducting secret marriages of military men in the year 270.
Claudius II decided that single men made better soldiers than those with wives and families, so he outlawed marriage for young, single men, who made up his military. Valentine defied Claudius and performed marriages for young couples in secret. When his actions were revealed, Claudius put him to death.
Another version of the story says that Valentine was a holy priest in Rome, who helped Christians escape harsh Roman prisons where they were often beaten and tortured.
Valentine was arrested and sent to the prefect of Rome for this. He found that his attempts to make Valentine renounce his faith were useless, and so recommended he be beaten with clubs, and later beheaded. This took place on February 14, 270.
According to the Catholic encyclopedia, there are at least three different Saint Valentines, all of whom are Christian martyrs of February 14.
Valentine’s Day on February 14 is just one of the many holidays which are often passed off as harmless and fun. But will it be strong to say that the Valentine’s Day is more than just about cards, cartoons and chocolates? It’s an initiation into the culture of boyfriends and girlfriends, sex outside of marriage, and the hurtful and often painful dating game!
It’s a way to plant the seed of "romance" in the minds of young people, with little or no emphasis on commitment and loyalty through marriage and a stable relationship with the opposite sex.
For Muslims, the implications of Valentine’s Day run even deeper. We have to educate our kids about Halal gender relations, in all that implies.
Muslims are first, brothers and sisters in faith, not sex objects to each other. Islamic rules of modesty and respect between the sexes need to be taught in this culture of low-cut shirts, tight jeans, mini-skirts, short shorts, seduction, phone-sex hotlines and the ever-present sexual humor and situations that pervade most sitcoms and television shows.
This need to abstain from Valentine’s Day is even more important for Muslims in the current global context, where Western secular culture is sneaking into Muslim societies via satellite dishes, television programs and various other forms of very powerful media. Most Muslim countries cannot or are not making enough efforts to resist this cultural onslaught and perhaps don’t fully comprehend the serious implications it has on Muslim children, families and gender relations.
http://soundvision.com/valentine/perspective.shtml