Many people say “happy belated birthday” if they’re telling you after it has passed. The word “belated” is an adjective meaning “Coming or happening later than should have been the case.” So, in the phrase “happy belated birthday”, “belated” is describing “birthday” as happening later than it should have. It’s not the birthday that’s late. It’s the person’s acknowledgement of it that is happening later than it should. This is one of those cases where people say something based on what most people think it means without truly understanding what they’re saying. The subject of this article would be a more appropriate use of the word.
Am I being picky? No. Just felt like educating. I don’t really care that much how you say something so trivial, but I thought you should understand what it is you’re saying.