St. Valentine's Day (commonly known just as Valentine's Day) is perhaps one of the best examples of a Christian holiday which has become secularized. It is generally associated with romantic love, and many people do not even know that it is the memorial of a saint.
St. Valentine was a priest in 3rd century Rome. There are many legends about him, but all that is known for certain is that, for the crime of being a Christian and trying to convert others, he was beaten with clubs and beheaded during the reign of Emperor Claudius II. Like most saints in the early Church, he began to be venerated shortly after his death, though his memorial did not become universal until the 5th century, when he was canonized by Pope Gelasius I.
How St. Valentine's Day became associated with romantic love is not entirely certain, though it likely occurred in the early middle ages. It is probable that his memorial was popularized as a replacement for the pagan feast of Lupercalia. His earthly remains are now displayed in St. Anton's Church in Madrid, though his relics can be found in churches all over Europe.
So, rather than treating it merely as an occasion for secular mirth, remember today to pray for the intercession of St. Valentine, that he may ask God to grant you a happy marriage on Earth and eternal bliss in the life to come.