Saint Valentine was a Roman priest whom was considered the patron Saint of lovers and was martyred for not giving up Christianity. Legend has it that when Rome was under the rule of the Emperor Claudius II that the Emperor passed a law that made it illegal to marry. The Emperor was a war monger whom expected men to volunteer for the army and fight for Rome, but most weren't interested and would rather live in peace with their wives and children, so the Emperor thought that if the men were not allowed to marry they would be more likely to join the army and go to war.
Saint Valentine being a priest who believed in holy matrimony would marry couples in secret and defy the newly written law. One night Valentine was caught performing a marriage ceremony and luckily the young couple managed to escape the soldiers but Valentine was not so fast and was captured and thrown in jail and told by the Emperor that his fate would be death for his traitorous disobedience.
Whilst Valentine was in jail he had many young couples visit him to thank him for his belief in marriage. One day a daughter of one of the guards at the jail came to visit him and did so frequently right up until his execution.
The legend is that he left the young lady a letter on the day he died thanking her for the company and all the kind words and signed it from: your Valentine; the day that Saint Valentine was killed was 14th February 269 AD.
It has been speculated that because of this; the tradition of exchanging love letters started on Valentine's day then developed into the custom of sending cards and exchanging gifts.