If you want to wish someone a happy birthday, but you forgot to do so on their actual birthday, you might use the phrase, “happy belated birthday.” Another way of saying it would be, “A belated happy birthday to you.” Either of these phrases is an appropriate way of offering good wishes to someone for whom you have missed their birthday.
Some might say that the phrase “happy belated birthday” is incorrect because it implies that the birthday has come late and not the wishes. However, this phrase has come to mean the same thing as “belated happy birthday,” and thus is correct to use in proper English. No one will think less of you, or scrutinize your language skills, for using either one of these phrases, unless they are an overly pedantic type.
In fact, these phrases are so commonly used in American English that greeting card companies sell a wide variety of birthday cards especially geared towards those who forget the birthdays of friends and family members. You can find belated birthday cards using any number of variations on these phrases in almost any retail store.
While some people may be upset that you missed their birthday, they are more than likely going to be happier that you wish them a belated birthday than to not say anything to them at all. Saying to a friend, “Happy belated birthday” shows that although you forgot initially, you later took the time to wish good things for them when you did remember.