This answer somewhat depends on which birthday it is asking about. In general, not much is done for Ender's birthday, because he is in Battle School. We are told that Ender figured out how to access a calendar, and he saw that the day was his birthday. The adults in Battle School also noticed, and they issued Ender a new uniform.
Ender turned seven. They weren't much for dates and calendars at the Battle School, but Ender had found out how to bring up the date on his desk, and he noticed his birthday. The school noticed it, too: they took his measurements and issued him a new Salamander uniform and a new flash suit for the battleroom. He went back to the barracks with the new clothing on.
Ender is a bit saddened by the lack of attention being given to his birthday, so he thinks about going to find Petra to tell her about home and birthdays there. On Ender's sixth birthday, Valentine baked him a cake. Unfortunately, the cake fell over and was described as "terrible." After Ender heads to Battle School, Valentine continues to remember and celebrate his birthday by lighting small fires:
Valentine celebrated Ender's eighth birthday alone, in the wooded back yard of their new home in Greensboro. She scraped a patch of ground bare of pine needles and leaves, and there scratched his name in the dirt with a twig. Then she made a small teepee of twigs and needles and lit a small fire. It made smoke that interwove with the branches and needles of the pine overhead. All the way into space, she said silently. All the way to the Battle School.