TOPSHOT - Britain's Queen Elizabeth II and Britain's Prince Philip, Duke of Edinburgh (R) travel in a horse-drawn carriage past Buckingham Palace on their way to Horse Guards Parade for the Queen's Birthday Parade, 'Trooping the Colour', in London on June 17, 2017.

The ceremony of Trooping the Colour is believed to have first been performed during the reign of King Charles II. In 1748, it was decided that the parade would be used to mark the official birthday of the Sovereign. More than 600 guardsmen and cavalry make up the parade, a celebration of the Sovereign's official birthday, although the Queen's actual birthday is on 21 April. / AFP PHOTO / Chris J Ratcliffe        (Photo credit should read CHRIS J RATCLIFFE/AFP/Getty Images)