Why was Infanta’s birthday a special day for her?

From the text:

Although she was a real Princess and the Infanta of Spain, she had only one birthday every year, just like the children of quite poor people, so it was naturally a matter of great importance to the whole country that she should have a really line day for the occasion.

On ordinary days she was only allowed to play with children of her own rank, so she had always to play alone, but her birthday was an exception, and the King had given orders that she was to invite any of her young friends whom she liked to come and amuse themselves with her.