Sadly it wasn't one birthday. Mine was near Christmas, so everybody remembered it (although I also hated it, because it was a dog and pony show to make everybody see what great parents they were), while my brother's was somewhere around April and thus was almost always forgotten.
I know someone whose parents didn’t forget their birthday, but consciously chose to downplay it because All Saints’ Day was the following day.
That is, they chose to make a bigger deal over celebrating the dead, than they did their living child.