Kate, Britain's Princess of Wales, arrived at Buckingham Palace on Saturday ahead of her first public appearance since surgery five months ago revealed the presence of cancer.
Broadcaster Sky News showed Kate, wearing a pale outfit, arriving in a car with her husband Prince William and their three children to attend "Trooping the Colour."
The annual military parade in central London marks the official birthday of the British monarch, King Charles.
Kate has undergone preventative chemotherapy since the cancer announcement, and while treatment is continuing, her improved health means she will be able to appear in public for the first time since last December.
Making progress but 'not out of thee woods'
In a rare personal written message on Friday, she said she was making good progress but was "not out of the woods." She said she was looking forward to attending Saturday's parade and hoped to join a few public engagements over the summer.
Kate spent two weeks in hospital in January after she underwent major abdominal surgery, and two months later she announced in a video message that tests had revealed the presence of cancer.
In her message on Friday, Kate said she had been "blown away" by thousands of kind messages from across the globe, which had made a world of difference to her and William, King Charles' heir.
"I am making good progress, but as anyone going through chemotherapy will know, there are good days and bad days," Kate's statement said.
Her office, Kensington Palace, has declined to give any details about the type of cancer or about her treatment, other than to say the preventative chemotherapy had begun in February.
Her illness has coincided with that of Charles, 75, who has also been undergoing treatment for cancer. He returned to public duties in April, and has remained busy, although his diary commitments are being limited to minimize risks to his recovery.