There are two ways in which the birthday dream is experienced.
The first is that everybody knows it's your birthday. There may be an actual party, or just a common affirmation and celebration
The second birthday experience is a darker image-you know it's your birthday and everyone forgot. It's an indication of the struggle for significance. Who you encounter, their apparent ignorance about your birthday, and their reaction to your news (or to your non-verbal, cloying attempt to clue them off) says a lot about the scenario. It may be that you feel overlooked in your relationships with others, or insignificant in general. If you are unwilling to tell, it may be that you are wanting more attention from the dream characters than they are providing. If, on the other hand, you tell them and they gush with apology, there may be a sense in which you feel they are operating at a relationship deficit with you over some matter. In the dream, did they give you a gift upon finding out it was your birthday? What was it?