While this is a meaningful day for Will and Kate's third child, July 9 already marks a memorable date for the Royal clan: It was on this day in 1947 that Queen Elizabeth and Prince Philip announced their engagement, and posed for photos at Buckingham Palace.
William and Harry wore matching blue suits.
The 36-year-old arrived hand-in-hand with her husband the Duke of Sussex ahead of the service at the Chapel Royal at St. James's Palace in London.
Despite still being officially on maternity leave, Kate Middleton attended the service as Patron of the RAF Air Cadets, which was her first official Royal military appointment, taking over from the Duke of Edinburgh in 2015.
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Dressed in a boatneck Ralph Lauren dress, she teamed it with a matching belt and a fascinator by Stephen Jones. She seems to be adoring American designers lately as the olive green quarter-sleeve dress is by Ralph Lauren.
Kate wore a pale mint Alexander McQueen design, while Meghan went dark. A few days ago, the Duchess of Sussex ditched her pastel outfits and wore a bright yellow ensemble while attending an event with Prince Harry. She also wore her hair in a low chignon, topped with a white, flowered headpiece.
While the princes have donned traditional uniform for the occasion, the ladies are wearing dress and hat combos.