Why do Ты know that Ginny's birthday is on August 11st? Did J.K. Rowling mentioned it in some interview? Beacuse it doesn't appear in the book.

I doubt about it beacuse, if Ты remember, Harry's disciplinary hearing was on August 12nd and Harry spent the last night (August 11st) in the Borrow, so Harry should know when is Ginny's birthday and would wish her happy birthday. And another thing, her family would have to organize a party for her, I guess, it would be the logical thing, I guess, isn't it? So, if Ты know it, answer it, please