Catherine will be out in public for the first time since Christmas Day. He said in March that he had cancer and was receiving treatment for it.
Princess Catherine expected on the balcony of Buckingham Palace along with the rest of the royal family.
They are to follow the king Charles’s military parade in honor of the official birthday.
Catherine was said to have arrived as a prince William’s and with his children to Buckingham Palace around one o’clock Finnish time.
On Friday, the princess published an open letter in which she said that her cancer treatments were going well.
The 42-year-old princess, known by the nickname Kate, is the most popular member of the British royal family.