Thousands of supporters will then arrive in Windsor for the wedding of Princess Eugenia and Jack Brooksbank.
Some 850 guests are expected, including X Factor judges Robbie Williams and his wife Ayda Field, singer Ellie Goulding and model Cara Delevingne.
Another 1,200 members of the public selected by vote will listen to a live broadcast on the grounds of the castle.
Many royal observers arrived in the city on Thursday to collect the best places for the procession of the newlyweds at noon.
The wedding will be shown in its entirety at ITV starting at 09:25 BST as part of a three-hour special of the daily program "This Morning".
The Princess Eugenia, who is the ninth in the row of the throne, is the daughter of the Duke of York and Sarah Ferguson, and the granddaughter of the queen.
She and drinks executive Jack Brooksbank will be married in St. George's Chapel, where Prince Harry and Meghan Markle were married in May.
The couple went to a final rehearsal at the site on Thursday, with photographers taking pictures of a smiling Eugenie in her car.
In an interview for This Morning, Eugenie admitted that the nerves before the wedding were "crawling".
He also described being "all butterflies" when they met, while skiing in Switzerland in 2010.
"I think I called my mother that night and I said, 'I met this guy, Jack … I remember I was like, really, I really like this guy, I really want him to also like me, "he said.
She realized that the feeling was mutual when he gave her a "huge wave of windshield wipers," he added.
Mr. Brooksbank described his future wife as a "bright and bright light".
"She has been amazing, she has the ability to do a million things at once in her brain, including work and organizing everything related to the wedding," he added.
Who will be there?
Among the celebrities expected to share their special day are supermodel Cindy Crawford, David and Victoria Beckham, singer James Blunt and his wife Sofia Wellesley, and George and Amal Clooney.
Members of the Royal Family are expected to begin arriving at the Galile Porch at Windsor Castle around 10:25 BST, and the ceremony will begin at 11:00.
The Duke and Duchess of the children of Cambridge, Prince George and Princess Charlotte, will assume once more the roles of godfather and maid of honor.
The other bridesmaids are Mia Tindall, daughter of Zara and Mike Tindall, cousins of Mia, Savannah and Isla, Eugenie's daughter, Maud Windsor and Robbie Williams' daughter, Theodora.
The other driver is Louis de Givenchy, six, the son of friends.
Eugenie's sister, Princess Beatriz, is a maid of honor. Mr. Brooksbank has chosen his brother, Thomas, as the best man.
What time is it happening?
- 07:00 – Doors of the castle open to the invited public.
- 08:30 – the guests begin to arrive at the chapel of San Jorge
- 10:32 – the mother of the bride – Sarah, Duchess of York – and Princess Beatriz arrive
- 10:35 – The boyfriend and the best man arrive
- 10:52 – the queen arrives
- 10:57 – the bride arrives, escorted by her father, the Duke of York
- 11:00 – the service begins
- 12:00 – boyfriends leave for the procession of carriages.
Italian tenor Andrea Bocelli will perform two pieces during the ceremony, which will also feature music by members of the Royal Philharmonic Orchestra.
You will also hear a personal prayer written by the Archbishop of York, Dr. John Sentamu.
& # 39; Perfect couple & # 39;
Windsor dean David Conner will officiate and give an address.
He described the queen's granddaughter and her fiance as the "perfect couple" after giving them the traditional pre-marital orientation.
"They are very natural among them, obviously they love each other, but they also have a lot of fun together and it has been a real pleasure to work with them," he said.
The queen is organizing a reception in the afternoon afterwards in the St. George's Hall in the castle. A party at night will be held at the York family home, Royal Lodge, in Windsor Great Park.
Families are paying for the private aspects of the wedding.
However, Republican activists say that taxpayers face a security bill of approximately 2 million pounds.
The campaign group Republic argues that, like Princess Eugenia, the ninth in the row of the throne, does not perform public tasks, families must pay the bill.
Buckingham Palace says this is a police matter.