The latest photograph is similar to the image released to mark that occasion, which was taken at a Norfolk beach.
Earlier this month, the family attended the annual Trooping the Colour ceremony, which celebrated King Charles's official birthday.
It was the first time Catherine - who continues to receive chemotherapy to treat her cancer - had appeared at a formal public event this year.
Days before the event, the princess released an update on her recovery, saying in a statement that she was making “good progress” and her treatment would continue for another few months.
"I am not out of the woods yet," she said, adding that there had been "good days and bad days".
The King is also being treated for an unspecified cancer and only made a return to public duties at the end of April.
On Friday, Prince William attended England's Euro 2024 match against Denmark in Germany.
He was pictured alongside Denmark's King Frederick in the Frankfurt stadium where the 1-1 draw was played.
