I found on a site called MissionIslam.com a rather extensive article on the topic. You can read the entire thing HERE. A few of the things that stood our for me are as follows:
The story of the Festival of Love (Valentine’s Day)
The Festival of Love was one of the festivals of the pagan Romans, when paganism was the prevalent religion of the Romans more than seventeen centuries ago. In the pagan Roman concept, it was an expression of “spiritual love”. There were myths associated with this pagan festival of the Romans, which persisted with their Christian heirs. Among the most famous of these myths was the Roman belief that Romulus, the founder of Rome, was suckled one day by a she-wolf, which gave him strength and wisdom. The Romans used to celebrate this event in mid-February each year with a big festival. One of the rituals of this festival was the sacrifice of a dog and a goat. Two strong and muscular youths would daub the blood of the dog and goat onto their bodies, then they would wash the blood away with milk. After that there would be a great parade, with these two youths at its head, which would go about the streets. The two youths would have pieces of leather with which they would hit everyone who crossed their path. The Roman women would welcome these blows, because they believed that they could prevent or cure infertility.
The connection between Saint Valentine and this festival
Saint Valentine is a name which is given to two of the ancient “martyrs” of the Christian Church. It was said that there were two of them, or that there was only one, who died in Rome as the result of the persecution of the Gothic leader Claudius, c. 296 CE. In 350 CE, a church was built in Rome on the site of the place where he died, to perpetuate his memory. When the Romans embraced Christianity, they continued to celebrate the Feast of Love mentioned above, but they changed it from the pagan concept of “spiritual love” to another concept known as the “martyrs of love”, represented by Saint Valentine who had advocated love and peace, for which cause he was martyred, according to their claims. It was also called the Feast of Lovers, and Saint Valentine was considered to be the patron saint of lovers.
One of their false beliefs connected with this festival was that the names of girls who had reached marriageable age would be written on small rolls of paper and placed in a dish on a table. Then the young men who wanted to get married would be called, and each of them would pick a piece of paper. He would put himself at the service of the girl whose name he had drawn for one year, so that they could find out about one another. Then they would get married, or they would repeat the same process again on the day of the festival in the following year. The Christian clergy reacted against this tradition, which they considered to have a corrupting influence on the morals of young men and women. It was abolished in Italy, where it had been well-known, then it was revived in the eighteenth and nineteenth centuries, when in some western countries there appeared shops which sold small books called “Valentine’s books”, which contained love poems, from which the one who wanted to send a greeting to his sweetheart could choose. They also contained suggestions for writing love letters.
Someone may ask: why do we Muslims not celebrate this festival?
This question may be answered in several ways:
1. In Islam, the festivals are clearly defined and well established, and no additions or subtractions may be accepted. They are an essential part of our worship and there is no room for ijtihaad or personal opinion. They have been prescribed for us by Allah and His Messenger (peace and blessings of Allah be upon him). Ibn Taymiyah (may Allah have mercy on him) said: “Festivals are part of the laws, clear way and religious ceremonies of which Allah says (interpretation of the meaning): ‘To each among you, We have prescribed a law and a clear way [al-Maa’idah 5:48] ‘For every nation We have ordained religious ceremonies which they must follow’ [al-Hajj 22:67] like the qiblah (direction faced in prayer), prayer and fasting. So there is no difference between joining them in their festival and joining them in their other rituals. Agreeing with the whole festival is agreeing with kufr. Agreeing with some of their minor issues is the same as agreeing with them in some of the branches of kufr.
Festivals are the most distinctive things by which religions are told apart, so whoever celebrates their festivals is agreeing with the most distinctive rituals of kufr. Undoubtedly going along with them in their festivals may in some cases lead to kufr. Dabbling in these things, at the very least, is a sin. The Prophet (peace and blessings of Allah be upon him) referred to the fact that every nation has its own festivals when he said: ‘Every nation has its own Eid and this is our Eid’ (al-Bukhaari , 952, Muslim, 1892).” (al-Iqtidaa’, 1/471-472)
Because Valentine’s Day goes back to Roman times, not Islamic times, this means that it is something which belongs exclusively to the Christians, not to Islam, and the Muslims have no share and no part in it. If every nation has its own festivals, as the Prophet (peace and blessings of Allah be upon him) said – “Every nation has its Eid” (narrated by al-Bukhaari and Muslim) – then this means that every nation should be distinguished by its festivals. If the Christians have a festival and the Jews have a festival, which belongs exclusively to them, then no Muslim should join in with them, just as he does not share their religion or their direction of prayer.
2. Celebrating Valentine’s Day means resembling or imitating the pagan Romans, then the Christian People of the Book in their imitation of the Romans in something that was not a part of their religion. If it is not allowed to imitate the Christians in things that really are part of their religion – but not part of our religion – then how about things which they have innovated in their religion in imitation of idol-worshippers?!
Imitating the kuffaar in general –whether they are idol-worshippers or People of the Book – is haraam, whether that imitation is of their worship – which is the most serious form – or of their customs and behaviour. This is indicated by the Qur'an, Sunnah and ijmaa’ (scholarly consensus)
Mashallah that really seems to cover it. There will be people that ask what is the harm? We don't celebrate it now like they did then? But to be honest, who cares? Why do we even need to participate in ridiculous things like this? We were given days to celebrate in Islam and they are the best of days.