It will be the most important day of their shared lives to date, imbued with centuries-old tradition as they take vows before God, their families and the Queen.
But not for Prince Harry and Meghan Markle a day of too much solemnity, as they promise a wedding focused on fun, joy and a chance to celebrate with the public.
The couple, who announced their engagement in November, are planning their own wedding celebration that "reflects the characters of the bride and groom".
From flowers to seating plans, the happy couple are taking charge of a ceremony designed to incorporate both families, their wide circle of friends, and welcome the public into their lives.
Here's all the latest news about their big day.
When is the big day and who is planning it?
The wedding is on Saturday, May 19 at midday at St George's Chapel, Windsor Castle, where Prince Harry was christened. It is said to have become a "very special place" for the couple in their relationship so far.
St George's Chapel last hosted a royal wedding in May 2008, when Peter Phillips, son of the Princess Royal, married Autumn Kelly. Prince Edward also wed Sophie, the Countess of Wessex, there in June 1999.
The Lord Chamberlain's Office at Buckingham Palace is responsible for organising royal weddings, but Harry and Markle are leading the show.
Under the guidance of Harry's private secretary, Edward Lane Fox, the Lord Chamberlain's Office will deal with the ceremonial aspects of the day.
The "core aspects" of the wedding, including the church service, associated music, flowers, decorations and reception, will be paid for by the royal family. Markle's parents are set to attend.
Former Suits star Markle told last year how she could not wait to say "yes" to the prince when Harry got down on one knee as they cooked a roast chicken dinner at his home, Nottingham Cottage at Kensington Palace.
Indicating the depths of his feeling for his bride-to-be, the prince said in a television interview: "The fact that I fell in love with Meghan so incredibly quickly was sort of confirmation to me that everything, all the stars were aligned, everything was just perfect.
"It was this beautiful woman just sort of literally tripped and fell into my life, I fell into her life."
A few months into the relationship, Harry said he had to have "some pretty frank conversations" with his girlfriend about how her life may change when she entered the royal family.
He added: "But I know that at the end of the day she chooses me, and I choose her, and therefore whatever we have to tackle together or individually we'll always be us together as a team, so I think she's capable. She's capable of anything."
Invitations - who is on the guest list?
Invitations are likely to be sent soon to about 800 guests, which is the total number that can be seated inside St George's Chapel.
Markle's father Thomas has not yet met Prince Harry in person, but he gave his blessing for the wedding and is expected to attend with Doria Ragland, the future mother-in-law the prince described as "amazing".
US president Donald Trump is not expecting a coveted invite, but some of those who might be invited include former US president Barack Obama, tennis star Serena Williams and Markle's Suits co-stars Sarah Rafferty, Patrick J. Adams and Wendell Pierce.
In 2011, Prince William and Kate Middleton's invitations - traditional white cards, gilded along the edge with gold and the royal cypher - were sent about 10 weeks before their wedding.
Harry and Markle have about nine weeks until their big day.
They may opt for the traditional royal invitations on thick white card, which begin with the words "The Lord Chamberlain is commanded by The Queen to invite ...".
But Markle is an expert calligrapher, who used to craft invitations for celebrity weddings, so may decide on her own alternative style.
What about the best man and bridesmaids?
The Duke of Cambridge is expected to be Harry's best man, or perhaps jointly take on the task with one of Harry's childhood friends. William has already joked that he is yet to be asked.
Prince George and Princess Charlotte look almost certain to be page boy and flower girl. The siblings already know the score - they performed those roles at the wedding of Pippa Middleton, the Duchess of Cambridge's younger sister, to James Matthews in May last year.
Markle may also choose grown-up bridesmaids from her close set of friends - most likely Canadian stylist Jessica Mulroney and Indian actress and philanthropist Priyanka Chopra. She might even pick a best woman.
Mulroney and Markle have been firm friends since meeting in Canada where Markle filmed several series of Suits, and bonding over a shared love of yoga (the duo both practice regularly).
Christening and Confirmation
Meghan Markle has been baptised into the Church of England in a ceremony with Prince Harry at her side.
The intimate 45-minute service was conducted by the Archbishop of Canterbury at the Chapel Royal on Tuesday evening.
Her baptism was followed immediately by her confirmation, which means she will be able to join Harry, 33, at Holy Communion.
Markle did not need to become an Anglican to marry Harry in church, but at the time of their engagement last November she made clear she had chosen to be baptised and confirmed out of respect for the Queen's role as the head of the Church of England.
Mulroney runs a bridal store in Toronto so is said to have been helping the bride choose her gown.
Erdem is among the contenders and is known to be among Markle's favourite designers. She also told Vanity Fair that Erdem is "a designer I've been wearing for years" when she wore one of his dresses for her interview with the title.
The Canadian-Turkish designer is known for his ladylike designs, which take their inspiration from art, literature and history.
Harry and his bride-to-be will be deciding on their chosen music, hymns and prayers for the traditional ceremony at St George's Chapel, which will be conducted by The Dean of Windsor, The Right Reverend David Conner.
The Archbishop of Canterbury, The Most Reverend and Right Honourable Justin Welby, will, as expected, officiate as the couple make their marriage vows.
Markle, who was a UN women's advocate, is unlikely to opt to obey the prince. They will probably choose the Series One (1966) Book of Common Prayer ceremony, just as William and Kate did, which allows the bride to drop "obey him" and "serve him" from the religious proceedings.
Harry also has to decide whether to wear a wedding ring. William does not wear one.
After the ceremony, among the first people to congratulate the couple will be some of the 2640 charity workers, community champions and local school children who are being invited to watch the wedding from inside the walls of Windsor Castle.
At 1pm, the newlyweds will then tour Windsor as their first trip as man and wife, to greet well-wishers, leaving Windsor Castle by carriage, via Castle Hill, along the High Street and through Windsor Town before returning along the Long Walk for their reception.
After the carriage ride around Windsor, the congregation will await the happy couple in St George's Hall, to celebrate with the first of two receptions. The second, for close friends and family, will be thrown in the evening by Prince Harry's father, the Prince of Wales.
From the food to the cake and the seating plan, there is much to decide on for the wedding breakfast - the meal following the ceremony - and the evening celebrations.
In December, it emerged the couple were considering a wedding cake made from bananas. Prince Harry is said to love "anything with banana".
The decision would follow a similar pattern set by the Duke and Duchess, who had a groom's cake made of chocolate biscuits for their reception at Buckingham Palace as well as a traditional eight-tiered cake.
Peonies are one of Markle's favourite flowers, so it is likely her floral bouquet will include the fragrant blooms, which are in season in May.
As is tradition for royal brides, there is also expected to be a sprig of myrtle, grown from the original myrtle in Queen Victoria's wedding bouquet.
The prince will be putting pen to paper to write his speech for their reception.
But Markle, who is an accomplished public speaker, is expected to break with tradition and deliver her own, offering "affectionate" praise of her new husband, thanks to the Queen, and a few suitable jokes.
The former actress is "ready" to speak at the reception, should her father Thomas Markle choose to avoid the limelight, the Sunday Times reported.
Thomas Markle, an award-winning Hollywood writing director who now lives a quiet life in Mexico, is expected to walk Markle down the aisle, but may not make a formal speech in front of hundreds of guests and senior members of the royal family.
At the wedding of Prince William and Kate Middleton, reception speeches included words from best man Prince Harry, the groom's father the Prince of Wales and Michael Middleton, who affectionately described how he knew his daughter's relationship was serious when her boyfriend landed a helicopter in his garden.
In February Mel B, one of the five original Spice Girls, hinted that the Spice Girls were expected to reunite for the royal wedding, confirming that they were to attend.
The singer, known to fans as "Scary Spice", claimed she had received a "proper" invitation, appearing to disclose the band are set to perform, during an appearance on a television chat show.
Sir Elton John has reportedly cancelled two dates of his Las Vegas residency to attend the royal wedding, amid speculation he could also be performing.
Plans will already be in place for policing the grand royal occasion, as thousands of people are expected to descend on the town of Windsor in Berkshire to catch a glimpse of the bride and groom.
Taxpayers pick up the large bill for security at the event.
The total cost of this wedding is still unknown, but Kate and Prince William's nuptials reportedly topped £10m ($19m), including the cost of policing and security.
New royal titles
Markle will become an HRH and a senior royal after marrying Harry and joining the Windsors. She is also expected to become a duchess, as Kate Middleton, who is expecting her third child in April, did after saying her vows in 2011.
By tradition, male members of the royal family receive a title from the monarch on their wedding day, and the vacant title Duke of Sussex is regarded as the most likely choice for the prince, meaning Markle would become HRH The Duchess of Sussex.
Like Prince Harry, the previous Duke of Sussex lived at Kensington Palace, and he also married for love, rather than choosing brides who fitted the traditional royal mould.
Will there be a bank holiday?
The wedding is on a Saturday, so no new bank holiday will be declared to allow the public to celebrate the wedding.
Although the public were given a day off to celebrate the weddings of Prince Charles to Diana Spencer in 1981, Prince William to Catherine Middleton in 2011, and Princess Anne to Mark Phillips in 1973, no national holidays were awarded for Prince Andrew or Prince Edward.
However, it's not all bad news because pubs and bars will be allowed to do away with the usual curbs on opening hours on the wedding weekend.
Ministers plan to extend licensing hours across Britain to allow venues to remain open until 1am on the nights of Friday and Saturday May 18 and 19, which is also the day of the FA Cup final.
Souvenirs and coins
The royal bride and groom will have to approve Royal Collection wedding souvenirs and decide on a design for their cyphers and look at their Conjugal Coat of Arms.
The royal wedding souvenir phenomenon dates back to Queen Victoria's wedding to Prince Albert in 1840, linked to the rise of mass production in Victorian Britain.
It remains a thriving industry today. An estimated £222 million ($424m) was spent on memorabilia in the run-up to the 2011 royal wedding, according to the Centre for Retail Research.
To celebrate the wedding of the Duke and Duchess of Cambridge, a range of china personally approved by the couple was released. The Royal Mint also released a commemorative coin.
Where will the couple go on honeymoon?
The prince and Markle will be hoping their chosen destination stays top secret as they enjoy a break after the busy run-up to the wedding.
Botswana, where they holidayed in the early days of their whirlwind relationship, is a good bet, (Markle's engagement ring features diamonds from Diana's personal collection, and a stone from Botswana), but wherever they go, the newlyweds will be looking for seclusion away from the limelight as they start their married life together.