How was August 15 decided as India's Independence Day?

According to the book 'Freedom at Midnight', the last Viceroy of India, Lord Mountbatten, decided on August 15 as India's Independence Day impulsively at a press conference. He finalised the date, as it was the second anniversary of Japan's announcement of surrender in World War II. Notably, Emperor Hirohito's radio broadcast had announced Japan's surrender on August 15, 1945.