From the royal web site -
The Queen celebrates two birthdays each year: her actual birthday on 21 April and her official birthday on a Saturday in June.
Official celebrations to mark Sovereigns’ birthday have often been held on a day other than the actual birthday, particularly when the actual birthday has not been in the summer. King Edward VII, for example, was born on 9 November, but his official birthday was marked throughout his reign in May or June when there was a greater likelihood of good weather for the Birthday Parade, also known as Trooping the Colour.
Thanks az_gila.