He fell in love with a girl and has a relationship with her and wants to celebrate it
Question

I have a romantic relationship with a girl who I love very much and I hope to marry her soon, if Allah wills. My question is: On a certain day, which is the 15th of the blessed month of Ramadan every year, we call this day the day of joy and love. Can we celebrate this day as a happy occasion and regard it as a blessed day? Can we ask those whom we invite to the party to pray to Allah to bring us together in goodness and happiness?.
Praise be to Allah.You are asking about the ruling on your celebrating this occasion; why did you not ask about the ruling on your “romantic” relationship with a girl who is a non-mahram to you?! 
This is what is basically wrong in your situation, this haraam relationship with a girl who is a non-mahram to you. Your relationship with her will remain haraam unless she becomes your wife. So it is not permissible for you to be alone with her, or to spend time with her, or to look at her, or to enjoy talking to her, because she is a non-mahram to you. Fear Allah with regard to yourself and your religious commitment; do not betray yourself on the basis of illusions of joy and blessing! 
What joy, what blessings can there be when you and this girl are disobeying Allah?! 
See the answer to questions number 47405 and 59907, as well as the section on relationships between the sexes on this website, in which you will find a lot of answers about that. 
Once you know that this romantic relationship with a girl who is a non-mahram is a relationship which is haraam from the outset, then everything that is based on falsehood is also falsehood. 
We would go further and say: If the love relationship that you are speaking about was with a permissible wife, then celebrating this day that you are asking about would be reprehensible innovation. We have no Eids except Eid al-Fitr and Eid al-Adha, and our weekly ‘Eid, which is Jumu‘ah. 
Keep yourself busy, instead of celebrating love, by hastening to get married, if you are in love with her and she is suitable to be a Muslim wife to you who will adhere to the commands of Allah with you. 
We ask Allah to keep us and you safe from temptation, both apparent and hidden. 
For more information on celebrating Valentines Day, please see the answer to question number  73007. 
And Allah knows best.