I have always heard the expression "happy birthday". But in a scene of an episode of the series heroes I have heard "happy anniversary".
Is the expression happy anniversary used with exactly the same meaning as happy birthday? How common is it to use such expression?
Actually in the context of that heroes episode, this expression was used to congratulate a woman (Niki) who had achieved to stay some time (I don't remember if a year or a month) away from drugs. So I guess this expression happy anniversary is used in all contexts but birth anniversary. Is this correct?