The Princess of Wales said in March that she'd been diagnosed with cancer following abdominal surgery.
The Prince and Princess of Wales celebrated 13 years of marriage on Monday.
Louis Arthur Charles was born on April 23, 2018, at St. Mary's Hospital in London.
The queen was born Elizabeth Alexandra Mary in London on April 21, 1926.
The Prince of Wales stepped out for his first in-person engagement after the Princess of Wales shared her health update on March 22.
The Princess of Wales spoke about her diagnosis and treatment in a video released on Friday.
TMZ obtained footage of the couple visiting a business a mile from their home in Windsor over the weekend.
The Princess of Wales, who was photographed with her three children, also shared a personal message.
The Princess of Wales returned home to Windsor last month after spending nearly two weeks in the hospital.
King Charles III, who is being treated for an undisclosed form of cancer, also will not attend the service of the late King Constantine II of Greece.
