Valentine was a Christian priest who lived and worked in Rome during the 3rd century. At that time most people in Rome were not Christian.
Claudius II, the Roman Emperor was waging war. He didn't want young men to get married; he wanted them to join his army. He said that no one could get married without his permission [they had to ask him in person, and he had to say yes].
Many young couples wanted to get married without the Emperor's permission. Valentine married them in secret, turning them away from the old Roman religion and making them Christian.
When Claudius found out, he put Valentine in prison, where he was tortured [hurt for punishment]. Finally, Claudius had Valentine killed sometime around the middle of February 270 A.D.
There is a story – probably not true – about Valentine helping one of the guards in the prison. The guard was in love with a beautiful young girl but he was too shy to tell her. Valentine helped the guard write a love letter to the girl. When the girl read the letter, she fell in love with the guard.
About 200 years after Valentine's death, the Christians came to power in Rome. Many people became Christians, but they still celebrated the old festivals. The Festival of Lupercus was still very popular.
The Christian leaders were very unhappy that so many people were still celebrating pagan [a non-Christian religion with many different gods] festivals. They declared that Valentine was a Saint [a very good Christian who could go straight to Heaven], and started their own festival based around him.
They said that good Christians should only celebrate St. Valentine's Day and should not take part in the Festival of Lupercus. Eventually, St. Valentine's Day became a popular holiday throughout Western Europe, and recently, most of the world.
To read a Japanese translation of this post, click on: バレンタインズデーの始まりは？