This is the 2049th most frequent Russian word.
валентина
"Валентина" is a Russian female given name, equivalent to "Valentina" in English.
In this sentence, 'Валентина' is used as a proper noun functioning as the subject, referring to a person's name.
Валентина подарила нам книгу.
Valentina gave us a book.
Here, 'Валентина' is part of a set phrase (Святого Валентина), referring to Saint Valentine.
На день Святого Валентина мы обмениваемся открытками.
On Valentine's Day, we exchange cards.
In this context, 'валентине' is a grammatical form of 'Валентин', denoting Saint Valentine, in the prepositional case.