How do you say happy valentines day in Greek?

The most common and straightforward way to say "Happy Valentine's Day" in Greek is:

Χρόνια πολλά για την ημέρα του Αγίου Βαλεντίνου! (Chronia polla gia tin imera tou Agiou Valentinou!)

While "Χρόνια πολλά" (Chronia Polla) literally translates to "Many years," it's a common well-wishing phrase used for many holidays and occasions, making it a suitable and widely understood way to express the sentiment of "Happy Valentine's Day".