Valentine’s Day is for giving valentines to valentines.
What does Merriam-Webster have to say?
Merriam-Webster has two listings. One is valentine as a noun. This is the card or gift, or the person receiving the card or gift.
The second is Valentine’s Day as the holiday (also a noun), with the capital V and D and the apostrophe between e and s.
For what it’s worth, either of these could, like any noun, be used as an adjective as well. This is where things can get tricky in verbatim documents.
Q. Mr. Valentine, do you have any hobbies?
A. Ever since I can remember, I’ve collected valentine cards.
Q. What do you do with all the Valentine’s cards?
A. I give them to my valentine and now my valentine’s collecting them, too.
Q. Mrs. Valentine must be happy about that.
A. My valentine’s cards are really special to her.
Just to keep things interesting, how about when you address your valentine in direct address: “Hello, Valentine”? The capital V there could cause some debate.