King Charles found a special way to celebrate what would have been Queen Elizabeth’s 100th birthday.

On Tuesday, April 21, the late Queen’s centennial, King Charles and Queen Camilla hosted a reception at Buckingham Palace commemorating the historic day. Invitees included representatives from some of the late Queen’s many patronages, including Cancer Research U.K., British Red Cross Society, Jockey Club, Royal Kennel Club and the Army Benevolent Fund.

Many members of the royal family joined the palace reception, including Kate Middleton, Prince William, Princess Anne, Prince Edward and Sophie (the Duke and Duchess of Edinburgh), the Duke and Duchess of Gloucester, Princess Alexandra and the Duke of Kent.

The guest list also included other centenarians celebrating their 100th birthdays. The group celebrated with a birthday cake, which was decorated with the number “100” in gold, and the King presented them with their centenary cards in person.

Centenary cards are free messages from the King and Queen available to all British nationals. Messages are sent for 100th birthdays, 105th birthdays and each year after 105. Royal well wishes are also sent out for couples celebrating their 60th, 65th and 70th wedding anniversaries, and each subsequent year after 70.

King Charles sent out his first centenary cards in October 2022, following the period of mourning for the late Queen Elizabeth. The cards featured a photo of the King and Queen Camilla, as well as a personalized message for the recipient.

“My wife and I are so pleased to know that you celebrated your one hundredth birthday on [personalized date],” the card read. “This brings our warmest congratulations and heartfelt good wishes on such a special occasion.”

The King also recognized his late mother on Tuesday with a pre-recorded video message.

Additionally, he and Queen Camilla visited the new Royal Collection Trust exhibit, Queen Elizabeth II: Her Life in Style, at the King’s Gallery at Buckingham Palace. The display is the largest-ever exhibit of the late Queen’s fashion, jewelry and accessories, featuring pieces from throughout her life, including some items never publicly displayed before.

The King’s sister, Princess Anne, 75, also marked the occasion by officially opening The Queen Elizabeth II Garden in Regent’s Park, London. Designed as a tranquil and fully accessible space for reflection in the heart of bustling London, the park honoring the late Queen opens to the public on April 27.

King Charles acceded to the throne upon his mother’s death on Sept. 8, 2022. The very next day, he gave his first speech as sovereign, in which he celebrated the late Queen’s devotion to country and family.

“Throughout her life, Her Majesty The Queen – my beloved Mother – was an inspiration and example to me and to all my family, and we owe her the most heartfelt debt any family can owe to their mother; for her love, affection, guidance, understanding and example. Queen Elizabeth was a life well lived; a promise with destiny kept and she is mourned most deeply in her passing,” he said, nodding to a pledge she made and her record 70-year reign. “That promise of lifelong service I renew to you all today.”

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The King pays tribute to his late mother each year on U.K. Mother’s Day, known as Mothering Sunday. This year, the tribute featured a throwback black-and-white photo of then-Prince Charles as a boy, sitting next to Queen Elizabeth on a bench in a garden. A young Princess Anne stands on the bench on the opposite side of her mother, with the queen’s arms wrapped around her legs.

“Wishing Mothers everywhere, and those who might be missing their Mums today, a restful Mothering Sunday,” the post’s caption read.