Valentine's day is basically a celebration of one or more christian saints called valentinus. All these martyrs are celebrated on February 14.
For instance, Valentine of Rome story goes like this: He was imprisoned for performing weddings for soldiers who were forbidden to marry and according to the legend, he also cured the daughter of the jailer. Just before his execution, he wrote her a farewell letter signed "Your Valentine". Saint Valentine's day is an official feast day for some communities of Christians. Some other communities also celebrate this feast day on July 6.
Valentine's day was later associated with romantic love and lovers started celebrating it all over the world with flowers, gifts, greeting cards, etc.