LONDON (AP) — King Charles III joined the queen and other members of the royal family for an Easter service at Windsor Castle on Sunday in what will be his most significant public appearance since he was diagnosed with cancer last month.

The monarch offered a cheery wave as he walked into St. George's Chapel for a service expected to last about an hour.

The appearance of the 75-year-old king is seen as an effort to reassure the public after Charles stepped back from public duties following an announcement by Buckingham Palace in early February that he was

undergoing treatment for an unspecified type of cancer.

Though he has been working behind the palace walls, his emergence in public for a royal fixture like the Easter service is seen as a positive sign.

The service itself will be smaller than usual as

Kate, the Princess of Wales

, is also being treated for cancer and has paused public duties. The princess, her husband Prince William and their children won’t attend the service.

Kate shock’s announcement that she, too, had cancer was made on March 22, after weeks of speculation about her health and whereabouts following major abdominal surgery in February.

While the Easter service takes place in a chapel on the grounds of Windsor Castle, the news media will be able to observe the arrival of Charles and Queen Camilla.