The Cambridges will appear in public for the first time as a family of four today when Princess Charlotte is christened in front of the Queen and close family.
Large crowds are expected to gather for a glimpse of the nine-week-old at her baptism at the Church of St Mary Magdalene in Sandringham - walking distance from the family's Norfolk country mansion Anmer Hall.
Guests including the Princess's great-grandparents the Queen and the Duke of Edinburgh, grandparents the Prince of Wales and Carole and Michael Middleton, step-grandmother the Duchess of Cornwall and Kate's siblings Pippa and James Middleton will watch the traditional religious service.
The Cambridges will also bring along toddler son Prince George, who turns two later this month.
Charlotte's godparents - most of whom are likely to be close friends of William and Kate - will be announced ahead of the service and be among the small congregation in the 16th century church.
Although the ceremony, conducted by the Archbishop of Canterbury the Most Rev Justin Welby, is private, the Duke and Duchess have agreed to allow members of the public into the area outside the church known as the paddock to see the royals as they make their way to and from the venue.
Police urged people to arrive in time for the opening of the paddock gates at 3pm. The ceremony is due to start at 4.30pm.
Just like George's baptism, a number of senior royals are expected to be missing. In October 2013 when George was christened at the Chapel Royal, the Princess Royal, the Duke of York and the Earl of Wessex were not among the guests.
Charlotte and George's full-time live-in nanny Maria Teresa Turrion Borrallo will be on hand but the Princess's uncle, Prince Harry, will miss the event, having already left to spend three months in Africa.
The church is used by the Queen when she is in residence on her private estate and is where the royal family worship on Christmas Day each year.
Baby Charlotte, the star guest, is likely to make a rare appearance for the cameras - only the second glimpse of the Princess in public since she was born.
Mr Welby will give an address and will be supported by The Reverend Canon Jonathan Riviere, the rector of the Sandringham group of parishes.
Mario Testino has been asked by William and Kate to take the official photographs after the service, with the Queen expected to sit for the world renowned fashion photographer for the first official time as part of the christening party.
The Peruvian - the favourite photographer of Charlotte's late grandmother, Diana, Princess of Wales - has worked with William and Kate, the Prince of Wales, the Duchess of Cornwall and the late Princess, but never done an official shoot which included the Queen.
Princess Charlotte Elizabeth Diana of Cambridge, fourth in line to the throne, was born on May 2 at 8.34am, weighing 8lbs 3oz.
Just like George, Charlotte will be christened in a replica of the intricate lace and satin christening gown made for Queen Victoria's eldest daughter, Victoria, the Princess Royal, in 1841.
Royal babies are also christened using the ornate silver gilt Lily Font which is usually on show as part of the Crown Jewels at the Tower of London.