The celebrations Tuesday marked the second event in a busy week for Britain's royals, following Monday's christening and ahead of the Queen's meeting with US President Donald Trump at Windsor Castle on Friday. Archbishop of Canterbury Justin Welby highlighted how members of his own family have served and been killed in the RAF - drawing attention to his own father who flew in Burma.
The former Suits star was the picture of elegance in a navy-black custom-made dress from Dior and Stephen Jones hat, while the mother-of-three was resplendent in a light blue coatdress from Alexander McQueen.
Louis will wear the handmade replica of the Royal Christening Robe, which in 2008 replaced the original which had been worn for all royal christenings since that of Queen Victoria's eldest daughter in 1841.
Charles, Camilla, William, Harry and Meghan will also meet veterans, personnel, charity representatives and those from the civil service in a reception after the spectacle.
She took the royal salute at the ceremony as the Prince of Wales and the Duchess of Cornwall, the Duke of Cambridge and the Duke and Duchess of Sussex watched nearby.
The Duke and Duchess, as well as Catherine's sister Pippa Matthews (previously Middleton), attended the Carters' wedding in Suffolk.
Middleton, who is now five-months pregnant with her first child with husband James Matthews, hid her baby bump under the long-sleeved pleated gown. However, she did break out of the box by wearing an exaggerated knit headband, which was tucked into her low bun. Prince George and Princess Charlotte were in attendance sure not to miss their younger brother's big day.