Trooping the Colour is an old ceremony. It was done before and after every battle: a battalion would "troop" or march the colours through the ranks so that every man would see that the colours were intact. This ceremony has been kept through time but nowadays it is mainly ceremonial.
Trooping the Colour is also known as The Queen's Birthday Parade.
It is held annually in London on a Saturday in June. This year Trooping the Colour will be held on 15th June.
Since 1748, this parade has marked the Sovereign's official birthday. Although The Queen was born on 21st April, it has since long been the tradition to celebrate her birthday on a day in the summer, when good weather is expected.
The Queen has attended Trooping the Colour every year of her reign, except in 1955 when a national rail strike cancelled the event.
This is the last official public engagement of the Duchess before she gives birth.
Camilla, Duchess of Cornwall, Prince Charles, Princess Anne, Queen Elizabeth II, Prince Andrew, Prince Harry, Catherine and Prince William all stand on the balcony at Buckingham Palace to greet the crowds and watch the Royal Air Force flypast.