The countdown is on for Britain's biggest royal wedding in years: The union of Prince Harry and Meghan Markle.

The couple’s May 19 wedding will be a public affair, with TV cameras allowed inside the wedding ceremony and more than 2,000 members of the public invited to watch the carriage procession outside after the wedding.

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Questions remain though on everything from who will design Markle’s wedding dress to which, if any, of the couple’s celebrity friends like Serena Williams and former President Barack Obama will be in attendance.

Here is what we know so far.

Who will design Markle's dress?

Designer Roland Mouret, a native of France who now calls London home, now appears to be one of the favorites to design Markle's wedding gown.

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Mouret, a close friend of Markle's, has remained tight-lipped about whether he has received the commission as her designer.

Another top contender for designing Markle's gown is the design duo behind Ralph & Russo.

Australian-born designers Tamara Ralph and Michael Russo have already designed for Markle, creating the $75,000 dress she wore in her intimate engagement photos with Harry.

Markle had her first fitting with the unnamed designer in January at Kensington Palace, royal reporter Omid Scobie told ABC News at the time.

She is also expected to wear two gowns on her wedding day, one for the ceremony and a more casual gown for a private evening reception.

Who's expected in the wedding party.

Harry and Markle have not yet publicly identified the members of their wedding party.

Prince William and Prince Kate's children -- Prince George and Princess Charlotte -- are expected to take center stage at the wedding as page boy and flower girl, respectively.

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Also expected to join Charlotte as a flower girl is Ivy Mulroney, the young daughter of one of Markle's closest friends, Jessica Mulroney. Harry and Markle may also follow in William and Kate's footsteps and choose some of their own godchildren to serve as bridesmaids and page boys.

British royal wedding tradition holds that bridesmaids are of grade-school age and are fellow royals. The groom’s page boys are also traditionally fellow royals or sons of aristocratic friends.

Best man and matron of honor

While Kate chose her sister, Pippa Middleton, as maid of honor, Markle has a small circle of close friends who could be by her side on her big day.

Mulroney, a stylist from Canada, is one friend who could play a big role in Markle's wedding day.

Another friend, Lindsay Roth Jordan, whom Markle knows from Northwestern University, could also be selected for matron of honor duties. Markle served as Jordan's maid of honor at her 2016 wedding in New York. Jordan was also one of the small group of women who attended Markle's bridal shower and is understood to have attended a dinner for family and close friends after Markle's baptism.

Heather Dorak, who runs a Los Angeles pilates studio, is another longtime friend of Markle’s who could take a role supporting Markle. Designer Misha Nonoo and Markus Anderson are two of Markle's closest friends who helped her relationship with Harry flourish and could be involved in the wedding party.

Harry and Markle may also decide to incorporate a reading at their ceremony, as William and Kate did with Kate's brother, James Middleton, at their wedding. One of Markle's tight-knit group of close friends might be considered for this honor.

Harry served as best man at William and Kate's wedding in 2011.

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William said in January that Harry had not yet asked him to be best man, before jokingly adding, "It could be a sensitive issue."

Harry has a group of longtime friends, including Guy Pelly, Tom Inskip, Arthur Landon and Tom Straubenzee, who could be by his side at the wedding. Capt. Mark Dyer, a mentor and friend to Harry, and his American-born wife, Amanda Kline, will also be among those making sure Harry’s wedding is special.

Location, location, location.

Harry and Markle will wed at St. George’s Chapel on the grounds of Windsor Castle, about 22 miles outside of London.

St. George's Chapel is a smaller venue than Westminster Abbey, where Prince William and Kate tied the knot in 2011, and St. Paul's Cathedral, where Prince Charles and Diana wed in 1981.

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The chapel, which can seat 800 people, is where Harry was baptized in 1984. It is also where Harry's father, Prince Charles, and Camilla, Duchess of Cornwall, held a service of prayer and dedication at the chapel in 2005 following their marriage at Guild Hall.

The last royal wedding to be held at St. George's Chapel was a decade ago, when Peter Phillips wed Autumn Kelly in May 2008, according to the chapel's website.

Wedding day lineup.

The wedding will begin at noon local time, 7 a.m., ET, on May 19.

The Right Rev. David Conner, the dean of Windsor, will conduct the wedding service. The Most Rev. Justin Welby, the archbishop of Canterbury, who baptized and confirmed Markle this month, will officiate when the couple takes their marriage vows.

Following the hour-long wedding ceremony, Harry and Markle plan to undertake a carriage procession that will start at St. George's Chapel, leave Windsor Castle via Castle Hill, continue along High Street through Windsor Town before returning to Windsor Castle on a route called the Long Walk.

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After the carriage procession, the newlyweds will attend a reception at St. George's Hall, where they will rejoin their wedding guests. The reception will be closed to the public.

Harry and Markle are expected to continue celebrating their marriage at an evening reception hosted by Harry's father, Prince Charles, for close friends and family. Details of the reception have not been released.

Will both of Markle's parents attend?

Harry's grandparents, Queen Elizabeth II and Prince Philip; his father, Prince Charles, the prince of Wales, and Camilla, duchess of Cornwall; his big brother, William, and sister-in-law, Kate, and their children, Prince George and Princess Charlotte, will be among the members of the royal family at the wedding.

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What role Markle's parents, Tom Markle and Doria Ragland, will play in the wedding remains to be seen.

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There has been speculation about whether Markle's father, mother, or both, would walk her down the aisle. Markle's father, who is divorced from her mother, spoke earlier this year about his excitement about Markle's upcoming wedding.

"I think it’s wonderful. I’m very delighted. I think they’re [a] very good match," Tom Markle said in a video obtained by the U.K.'s The Sun. "I’m very happy for them, Meghan and Harry. I love my daughter very much. Harry’s a gentleman.”

Princess Diana's sentimental influence.

Both Markle and Harry have said how important it is that Harry's late mother be included in their wedding.

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"I think she would be over the moon, jumping up and down, so excited for me," Harry said when his engagement to Markle was announced. "But then, as I said, [she] would've probably been best friends with Meghan."

Markle's engagement ring features two diamonds from Diana's personal collection.

"The little diamonds on either side are from my mother's jewelry collection to make sure that she's with us on this crazy journey together," Harry said in November.

There has also been speculation that Markle could choose between two of Princess Diana's favorite designers, Catherine Walker and Amanda Wakeley, both of whom have couture shops near Kensington Palace, to design her wedding dress.

The most celeb-studded royal wedding to date?

Among the high-profile guests who could attend Harry and Markle's wedding at Windsor Castle are Barack and Michelle Obama, Canadian Prime Minister Justin Trudeau and first lady Sophie Gregoire Trudeau and some of Markle's celebrity friends, including actress Priyanka Chopra and tennis star Serena Williams.

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Markle has also invited several members of the cast of "Suits," the TV show she starred on for seven seasons, ABC News has learned.

Also expected to attend the wedding ceremony is Elton John, one of Diana’s closest friends who has worked with Harry on AIDs policy.

Singers Ed Sheeran and Rihanna and members of Coldplay, who have all actively supported Harry's charitable work, could also get invites.

Spice Girls star Mel B said on a talk show in February that all five Spice Girls have been invited to Harry and Markle's wedding.

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Mel B declined to comment on whether the Spice Girls would perform on the big day, saying, "You’re going to get me fired! Let’s not talk about it anymore! Let’s pretend that I never said that.”

Other members of the girl band have since indicated they will not be performing.

Prince William and Princess Kate invited Ellie Goulding to perform at their wedding reception in 2011.

Everyday people will play a role.

Harry and Markle will invite 2,640 people onto the grounds of Windsor Castle to see the wedding carriage procession as it departs, Kensington Palace announced in March.

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Members of the public will be selected from different regions of the U.K. with a special emphasis on those who have served their community. The couple will also invite 100 students from two local schools in Windsor that have a strong affiliation with Windsor Castle.

Harry and Markle also plan to extend invitations to 200 individuals who take part in charities and organizations for which Harry serves as royal patron.

Is Markle planning this all on her own?

Markle's best friend and stylist, Jessica Mulroney, is also a bridal planner and assisting Markle in the entire design and concept of the wedding party outfits and look for the reception.

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Mulroney is Canada’s best-known stylist who styled Markle during her seven-season run on the TV drama "Suits," which films in Toronto.

Since Markle's engagement to Harry, Mulroney helped select the dress and coat Markle wore on the day the couple's engagement was announced in November. Mulroney was also at Markle's side advising her on the gown and sweater Markle wore for her intimate engagement photos with Harry.

Mulroney is the wife of Ben Mulroney, whose father is a former prime minister of Canada. The two are one of Canada's best-known power couples and are often considered Canadian royalty in Toronto.

Who is paying for this celebration?

Kensington Palace confirmed in a statement released after the couple’s engagement that Harry’s family would pay for the “core aspects of the wedding.”

"As was the case with the wedding of The Duke and Duchess of Cambridge, The Royal Family will pay for the core aspects of the wedding, such as the church service, the associated music, flowers, decorations, and the reception afterwards,” the statement read.

Markle is expected to pay for her wedding dress. Security costs for the wedding day will come at the expense of U.K. taxpayers.

How you can watch the wedding.

Harry and Markle’s wedding will be televised. Cameras will be allowed inside St. George’s Chapel for the ceremony and cameras will also line the carriage procession route in Windsor.

The receptions following the wedding ceremony will be private.

Stay tuned to ABC News as more details on the broadcast schedule and ABC News' coverage are announced.