Why Kate Middleton's Trooping the Colour Appearance Is a 'Positive' Surprise (Exclusive)

Kate Middleton announced on Friday she'll make her first public appearance in six months. She also offered a rare health update.

It's official. Kate Middleton will be taking part in one of the most important days in the royal calendar -- the Trooping of the Colour -- and that is "a very positive and happy surprise," royal expert Katie Nicholl tells ET.

The Trooping of the Colour -- a 260-year tradition marking the monarch's official birthday celebration that includes a massive parade comprised of soldiers, horses and musicians -- is embedded in the fabric of British society, much like crumpets and tea in the afternoon. That the Princess of Wales has made such progress amid her cancer battle that she feels well enough to attend is huge news, to say the least.

"I think it was a surprise for most of us that Katherine has announced she's going to be taking part in Trooping [of the Colour]," Nicholl tells ET. "There has been a bit of speculation that she might be well enough to be on the balcony. As ET has reported, we know that the Princess of Wales has turned a corner in her treatment, and we know that she's going to be on the Buckingham Palace balcony. And I think it's a surprise and a very positive and happy surprise, too."

The mother of three announced on Friday that that her cancer treatment is ongoing and that she will be undergoing treatment "for a few more months." While she added that she's looking forward to attending the king's birthday parade with her family this weekend, Kate also expressed caution full well knowing she's "not out of the woods yet."

Kate's last public appearance came in December when she and her family -- husband Prince William and children Prince George, 10, Princess Charlotte, 9, and Prince Louis, 6 -- attended the Christmas Day service in the U.K. From that day forward, there was constant speculation as to her whereabouts. That is, until Kate announced her cancer diagnosis on March 22. Amid her recovery, the royal family has kept a busy schedule. William recently visited Normandy, where alongside President Joe Biden and Canadian Prime Minister Justin Trudeau and other dignitaries, he attended a ceremony to commemorate the 80th anniversary of D-Day at Juno Beach. William's appearance may have offered a clue that Kate was improving.

Canadian Prime Minister Justin Trudeau, U.S. First Lady Jill Biden, U.S. President Joe Biden, French President Emmanuel Macron, Prince William, look on as they attend the International commemorative ceremony at Omaha Beach marking the 80th anniversary of the World War II "D-Day" Allied landings in Normandy, Saint-Laurent-sur-Mer, France. - Getty

"The fact that William was able to go and do those D-Day commemorations in Normandy, I think they signal that at home things were getting better," Nicholl says. 

Trooping the Colour is annually a global event, but with Kate announcing she'll be attending -- which will will mark her first public appearance in six months -- the world will now be watching even more intensely. And what they'll see, for the first time in quite some time, is a sense of stability that's been missing amid Kate's cancer diagnosis as well as Charles' cancer battle. According to Nicholl, having both the monarch and Kate will convey an important message.

"It's a very important image to project to the world," Nicholl says of Kate's planned balcony appearance Saturday. 

Kate Middleton's new portrait. - Matt Porteous

She adds, "So, up there on that balcony you are seeing the royal family together, despite having gone through a very turbulent time. And I think that's going to send a really important message."

Kate's health update was also accompanied by a new portrait, making her first new photo since announcing her cancer diagnosis. In the portrait, Kate leans against a lush tree dressed in jeans and a beige blazer. The photo offered more clues as well.

"We now have heard for the first time since her cancer diagnosis from the princess herself in a message that she wrote herself alongside a newly-released picture," Nicholl says, "in which, I have to say, she looks incredibly healthy and very, very well."