Happy Belated Birthday

Как сказать "Happy Belated Birthday!" по-русски?

Phrase "happy birthday" doesn't work in russian - I do not know why. We can say "счастливого рождества!" ("happy cristmas" literally), but "happy" just is not used with "birthday".
Generally we say "С днём рождения!" which is common shortage of "Поздравляю тебя/вас с днём рождения".
Role of word "belated" is unclear to me in your example. If it is about lateness of congratulation you can say "С прошедшим днём рождения!".

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Originally Posted by Alex80

Role of word "belated" is unclear to me in your example. If it is about lateness of congratulation you can say "С прошедшим днём рождения!".

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Yes, I have forgotten to congratulate my Russian friend on the very day of her birthday, so want to say it now.
Thanks for your answer.))

You can even say just «С прошедшим!».