It is the country’s oldest and most senior Order of Chivalry, with the day set to see Ladies and Knights of the Order process down the hill from Windsor Castle’s State Apartments to St George’s Chapel, dressed in white plumed hats and dark blue velvet robes, watched by crowds of onlookers.
It is only outranked by the Victoria Cross and the George Cross.
The Order includes the King, who is Sovereign of the Garter, Queen Camilla, several members of the Royal Family, and up to 24 Companions who have been recognised for their dedication to public service.
New appointments are formally invested on the day, in the Garter Throne Room, by the monarch.
There are currently 10 members of the Royal Family who have become Royal Knights and Royal Ladies within the Order.
King Charles was made a Royal Knight Companion of the Order of the Garter in 1958, when he became the Prince of Wales.
Since Queen Elizabeth’s death in 2022, he became the Sovereign of the Most Noble Order of the Garter
Prince William became a Royal Knight in 2008 and was also the 1000th Knight ever created.
Other royals being made Knights and Ladies include the Duke of Kent in 1985, the Duke of Gloucester in 1997, the Princess Royal in 1994 and Princess Alexandra in 2003 as Royal Knight Companions, and the Duke of York and Duke of Edinburgh in 2006.
Queen Camilla was made Royal Lady of the Most Noble Order of the Garter in 2022.
Last year, the Duchess of Gloucester was invested as a Royal Lady Companion of the Garter.
The Princess of Wales and the Duchess of Edinburgh are not in the ancient Order and but watch the service and procession from St George’s Chapel, before taking part in a carriage procession around the castle grounds.