Valentines day was starter to celebrate St. Valentine. 

 

Excuse me if I omit some of the details but here it is in a nutshell.

 

The Romans decided that marriages would no longer be allowed.  Valentine was a clerygyman that continued to marry people in secret.  He died for his efforts to help people do the "right" thing....live as man and wife and not in sin.

Of course now  it is just another commercial holiday.  As Alexa mentioned earlier the majority of the people who celebrate don't have a clue what or how Valentine's day began or what it is really about.

I don't know where Qatarfalcon got his facts from but the basic reason that Muslims don't celebrate Valentines day is because it celebrates a Christian Saint.  Saint Valentine was the total opposite of his description....he helped men and woman live in holy matrimony and not live in sin. 

This is the reason that it was not allowed in Saudi Arabia when we lived there.  I saw on the news today that they cannot even sell anything that is "red" in color on Valentines day.  I know the florists where we lived (there was only one at the time) made a killing every Valentine's day.  He doubled his price on roses.