She wore a gorgeous pastel blue dress by Alexander McQueen with her RAF brooch pinned to her lapel. She was joined by husband Prince William and other members of the Royal family.
And despite still being on maternity leave, after giving birth to Prince Louis only 11 weeks ago, it is understood that Kate felt it was important to attend Tuesday's celebrations. This marks the first time the Duchess of Sussex has worn the French label.
Now at the christening of Prince Louis Middleton high hairstyle appeared in a bright dress from Alexander McQueen. She paired the look with a gorgeous brooch, a matching Sean Barrett hat and nude heels.More news: Anderson beats Isner in longest semi-final in history
She wore one to Princess Charlotte's christening in July 2015 and repeated the look for the Trooping of the Color in July 2016 and a commemoration ceremony in Belgium in 2017. When Walter Cullen, 3, met Meghan and Prince Harry during their visit to Dublin this week, the little boy reached out to touch the Duchess of Sussex's brunette locks before clasping his palm to his mouth in disbelief.
Royal experts have continued to draw comparisons between the two women, ever since Kate and Prince William announced their engagement in 2010.
The family has a long-standing commitment to the Royal Air Force with four generations of the family having served or trained with the United Kingdom's aerial warfare force, and many holding honorary ranks and patronages with RAF charities and associations.
Diana's favourite flower was the white rose, leading observers to think Kate was making a subtle tribute to her.