British monarchs have two birthdays a year. One - when the king or queen of Great Britain really were born, and the second - appointed. For example, Queen Elizabeth II was born on 21 April, but her official birthday celebrated on a Saturday in June. And if your real birthday queen spends with his family, then at least in the official birthday party arranged mass festivities with artillery salute and hanging flags, and in British embassies around the world held receptions. It was in the Queen's official birthday ceremony held Trooping the Colour - a colorful parade, which is regularly held since 1748.
Because of all these celebrations celebration official birthday of the Queen (if on a throne will be the king, the name of the festival changed to "official birthday of the King"), decided to celebrate the summer, because the summer is much higher chance of good weather, so necessary for the folk festivals .