First and foremost, it is important to realize that Valentine's day is a bit different in Japan.
Valentine's day in Japan is, just like western countries, on the 14th. But it is about the woman in the relationship doing things for the man in the relationship. This means making chocolates (yes, making. Some of my students talk about making chocolates), making cards, and just all around women-doing-stuff-for-man type things.
Some of the presents given out have nothing to do with love, either. The concept of "Giri" (pronounce the "i's" as "EE") comes into play - you owe your boss and other influential people a present because ... they're awesome, or something...
Don't worry, ladies, you'll get yours!
On March 14th, White Day (please ignore the irony of a white guy talking about it), the man reciprocates with lovely presents for the fair maiden.
Looking at "Double_Frick"'s post - her video answers a lot of the questions.
Most of the "happy~~~" greetings for certain days in Japan use the phrase "omedtou". I.e. Tanjyoubi Omedetou! (happy birthday) etc. But I'm not sure if this is used for valentine's day, as my fiance and I are both Americans, and I have to buy her presents for Valentine's AND White day!!!