How do you say happy valentine in Japanese?

Shiawase na barentainde.

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Technically correct, but a little unwieldy. 'omedetou' is used more often for holiday greetings (otanjoubi omedetou = happy birthday, akemashite omedetou = happy new years, etc), so maybe barentain omedetou.

All that said, you don't typically greet people there with 'happy Valentine's Day', the girls just give the boys chocolate/candy, then the boys reciprocate on 'white day' which is exactly one month later.