It's the huge royal event that dates back more than 200 years and celebrates the official birthday of the King or Queen.
The annual Trooping the Colour parade on Saturday will see King Charles mark his first official birthday since becoming King last September.
Hundreds of horses and soldiers are set to carry out complex battlefield drill manoeuvres to military music - and many parts of the day will remain as they have done for years under the late Queen, with some royals travelling in carriages from Buckingham Palace to Horse Guards Parade to watch the service.