Celebrated across the Commonwealth, the day is an opportunity for each of the institution's 53 member states to highlight the positive values of the family of nations.
Since their announcement, Prince Harry and Meghan Markle have been inseparable, being seen at plenty of royal appearances and it's even been reported that Meghan's mum recently flew in for her baptism into the Church of England.
The American actress, who intends to become a British citizen following the royal wedding on May 19, was far more straight-faced as she joined the congregation in singing the national anthem. "Through exchanging ideas, and seeing life from other perspectives, we grow in understanding and work more collaboratively towards a common future", she said in a message.
And while the two leading ladies appeared to have chosen quite different outfits - Meghan in a pure white coat and matching beret, and Kate in head-to-toe navy - there was one detail that was nearly identical.
In another shot, Megs and Harry team up with their Bizarro Land comrades and enjoy a black-tie dinner with Prince William (Burgess Abernethy) and Kate Middleton (Laura Mitchell). Liam Payne, meanwhile, gave a performance at the celebratory event.
LOVES it. Now, we may've loved Meghan's look over Kate's outfit, but we adore seeing William, Harry, and their ladies color coordinating so perfectly. The fashion designers had previously dressed the bride-to-be for her engagement shots with Prince Harry.
"She was delighted and said she was "very, very excited" she stayed chatting with us for quite a while - what a lovely, lovely lady".
"What a lovely, lovely lady".
The former "Suits" star donned a small white beret that sat slightly forward and askew on her raven locks.