Prince William Joins King Charles and Queen Camilla at Royal Ceremony — Sporting Ostrich Feather Hats!

Kate Middleton did not step out for the event, which she has been attending since 2008 when she and Prince William were dating

There's a royal reason that Prince William, King Charles and Queen Camilla are sporting ostrich feathers in their hats!

Just two days after celebrating the King's official birthday at Trooping the Colour, members of the royal family gathered at Windsor Castle for Garter Day on June 17. The Order of the Garter ceremony has been held annually since 1948, celebrating the oldest and most senior order of chivalry in the U.K.

Prince William, King Charles and Queen Camilla took part in the Garter Day procession wearing ceremonial garb: the order's blue velvet robes and plumed hats. In addition to royals and other members sporting the ensemble each year for the ceremony, the Prince of Wales wore the Order of the Garter vestment at his father's coronation in May 2023.

Princess Anne and Prince Edward also took part in the procession.

Missing from the historic event was Kate Middleton. The Princess of Wales, 42, regularly attends the ceremony in support of her husband, poignantly making her Garter Day debut in 2008 as Prince William's girlfriend when he received his Order of the Garter knighthood. Her attendance was significant as it showed she was being welcomed into the royal family fold, even before the couple's engagement two years later.

Princess Kate has largely remained out of the public eye since announcing her cancer diagnosis in March. However, she shared a health update on June 14 and attended Trooping the Colour on June 15, marking her first outing with the royal family since Christmas. The Princess of Wales said in her June 14 message that she is "starting to do a little work from home" and expressed a "hope to join a few public engagements over the summer," but no date has been set for her to fully return to her royal duties. Any upcoming outings will be based on when she feels able and under the guidance of her medical team.

Related: Every Royal Who Appeared on the Buckingham Palace Balcony at Trooping the Colour — Including Kate Middleton!

The day's main event is the monarch's investiture of new members with the Order of the Garter's insignia in the Throne Room of Windsor Castle. The sovereign hosts a lunch for the members and officers, and they then process on foot to a service at St. George's Chapel. New Companions are installed during the short service, and the King and members of the order then travel back to the Upper Ward of Windsor Castle.

At the event, Birgitte, the Duchess of Gloucester made modern royal history with her investiture as a Royal Lady of the Most Noble Order of the Garter. Prince Richard's wife became the first non-blood royal who is not married to the monarch or the heir to the throne to be appointed to the position.

The move opened the door for other royal spouses like Sophie, the Duchess of Edinburgh to be welcomed in the future. Sophie attended this year's event, watching the procession that included her husband, Prince Edward.

Other inductees announced on April 23, the traditional Garter appointment announcement day and feast day of England's patron saint, St. George, included Lord Andrew Lloyd Webber, Air Chief Marshal Lord Peach and the Right Honourable Lord Kakkar. The three have been invested as Knight Companions of the Most Noble Order of the Companion.

Members of the royal family who are currently in the Order of the Garter include Queen Camilla, Prince William, Princess Anne, Prince Edward and Prince Andrew, as well as Queen Elizabeth's cousins, the Duke of Kent and Princess Alexandra.

The Order of the Garter was established by King Edward III in 1348. The royal was inspired by the story of King Arthur and the Knights of the Round Table and set out to recognize his own group of honorable knights.

Membership within the Order of the Garter is exclusive to the monarch, who is the sovereign of the Order of the Garter, several members of the royal family and 24 knights selected in honor of their public work or personal service to the sovereign. Currently, there are 21 Companions out of a maximum of 24, the palace confirmed in the April 23 update.

The statement also revealed that Princess Kate, Prince William and Queen Camilla also received new appointments from King Charles on April 23.

While William and Camilla took over for historic royals — Prince William became the Great Master of the Most Honourable Order of the Bath while the Queen became the Grand Master and First or Principal Dame Grand Cross of the Most Excellent Order of the British Empire — the Princess of Wales was given a brand new position. The King appointed his "beloved daughter-in-law" as the first-ever Royal Companion of the Order of the Companions of Honour. The order was founded in 1917 by Queen Elizabeth's grandfather, King George V, to recognize achievements in the arts, sciences, medicine and public service over a long period of time. Membership is exclusive to 65 people at a time (plus honorary members from outside the Commonwealth) and the contingent currently includes Elton John, Anna Wintour and David Attenborough.

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This year, the Order of the Garter festivities fell two days after Trooping the Colour, the King's annual birthday parade organized by the Household Division of the British Army.

King Charles participated in a modified way this year in light of his cancer treatment, and conducted the review of troops from his spot in a carriage alongside Queen Camilla. In years past, including the first Trooping the Colour of his royal reign, the King saddled up on horseback for the epic procession through London.

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