Prince Louis wore the same christening gown as his siblings, which is a replica of the robe made for Queen Victoria's eldest daughter in 1841, which is now too delicate to be worn.
The Prince of Wales, Duchess of Cornwall, Duke of Cambridge, Duke of York, and Duke and Duchess of Sussex then attended a reception here.
Look familiar? It should, as it's the same Alexander McQueen design she wore three times before in a creamy white shade for Prince Harry's and Meghan Markle's wedding in May, Trooping the Colour in 2017 and Princess Charlotte's christening in 2015.
Kate Middleton wore her hair in a familiar style when she joined the royal family in London to celebrate the 100th anniversary of the UK's Royal Air Force.
Archbishop of Canterbury Justin Welby, the spiritual leader of the world's Anglicans, will lead the service at the Chapel Royal with close friends and family in attendance.
Prince Harry has paid around $10,000 for a first edition "Winnie-the-Pooh" for nephew Louis' christening.
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.
Unsurprisingly considering her adorable performance at the hospital when she went to meet her little brother, Princess Charlotte stole the show again.
Kensington Palace reports that following the service, Wills and Kate hosted a lovely tea for guests back at Clarence House.
In a video, the Duke of Cambridge can be seen sitting alongside wife Kate Middleton-who broke her maternity leave for a surprise appearance at the event.