However, it's more commonly used in a context like "Whose birthday is it?" or "Who is having a birthday?"
Here are some examples:
* ¿Quien cumple años hoy? (Who is celebrating a birthday today?)
* No sé quien cumple años. (I don't know who is having a birthday.)
* ¡Feliz cumpleaños a quien cumple años! (Happy birthday to whoever is celebrating!)