Star-spotting will be the name of the game as guests file into St George’s Chapel.
Meghan Markle’s former Suits colleagues Patrick J Adams, Gabriel Macht and actress wife Jacinda Barrett are all in the UK, and Sir Elton John and the Beckhams are also expected to be among the 600 guests.
2. The dress – and the tiara
The design of Meghan’s wedding gown has been kept under wraps, but Ralph & Russo are the rumoured designers.
Will the bride go traditional and demure, or opt for full-blown Hollywood glamour?
Royal fans are also waiting to see which tiara – if any – the future royal wears and whether it will be Diana, Princess of Wales’s famous Spencer heirloom.
3. Beatrice’s hat
When the Duke and Duchess of Cambridge married in 2011, Beatrice’s obscure hat became a viral sensation.
It was compared to a toilet seat, a turkey twizzler and even a giant pretzel.
At the time the Philip Treacy creation attracted derision, mirth and utter bafflement from royal watchers, but sporting Beatrice later auctioned it off on eBay for charity.
4. The bridesmaids and pageboys
With a gaggle of six bridesmaids and four pageboys, Prince George and Princess Charlotte among them, someone is sure to get up to mischief.
Asked if someone is in charge of making sure all the children behave themselves, the Archbishop of Canterbury, who is marrying the couple, said: “I’m glad to say that’s really not our problem. I’m sure there will be. It will be beautiful.”
5. The sermon
Bishop Michael Curry has travelled from the US to deliver the sermon.
He has a reputation as a passionate speaker and is the first African-American elected as presiding bishop and primate of the Episcopal Church.
He has been praised for his “infectious laughter and self-deprecating humour”.
It will be a nerve-wracking time for Harry and Meghan so there may be a few slip-ups on the day.
Harry’s mother Lady Diana Spencer got the Prince of Wales’s middle names the wrong way round at their wedding.
The Duke of Cambridge struggled to fit the ring on his bride’s finger.
Kate Middleton, now the Duchess of Cambridge, had apparently ordered it a size too small.
7. Fergie and the Duke
Sarah, Duchess of York divorced the Duke of York many years ago, but left the royal family under a cloud after being photographed having her toes sucked by her financial adviser.
Philip, 96, is said to have never forgiven her, and was once reported to have described her as “having no point”.
Will the pair encounter each other in the church – or will Fergie be made to stay well away from her former father-in-law?
8. The Markles
Thomas Markle will not be at the ceremony due to ill health, but some of his relatives will be in town – although they have not been invited to wedding.
Tracy Dooley, who is married to Meghan’s half brother, has arrived in the UK with sons Tyler and Thomas, and has already done a series of interviews.
Meghan’s half sister Samantha Markle, who is the US, has also not been invited, but is unlikely to keep quiet on Twitter during the proceedings.
9. The kiss
Will they or won’t they?
Photographers are hoping Harry and Meghan will kiss for the cameras on the West Steps of the chapel, surrounded by the royal family, shortly after becoming husband and wife.
10. A second dress?
Kate wore two dresses to her wedding – one to the ceremony and another to the evening do.
Meghan could do the same as she prepares to party the night away with 200 guests at Frogmore House in Windsor.