September 20, 1486 – Birth of Arthur, Prince of Wales, son of King Henry VII of England, at St. Swithin’s Priory in Winchester, England
Unofficial Royalty: Arthur, Prince of Wales
September 20, 1853 – Birth of Chulalongkorn the Great, King of Siam (Rama V) at the Grand Palace in Phra Nakhon, Siam (now Thailand)
Wikipedia: Chulalongkorn, King of Siam
September 20, 1925 – Birth of Ananda Mahidol, King of Thailand (Rama VIII) in Heidelberg, Germany
Anada Mahidol, the brother of his successor King Bhumibol Adulyadej of Thailand, was found dead after he was shot in his bedroom.
Wikipedia: Ananda Mahidol, King of Thailand