Here's are the top things to look out for.
1 The Dress: This will be the star attraction and woe betide any guest who attempts to upstage the bride. Rumoured to be by British brand 'Ralph and Russo' who famously designed Meghan's eye-watering €56,000 engagement dress, that'll be a drop in the ocean compared to the real thing. Meghan won't expect change out of €135,000 for The Gúna although, compared to the £250,000 Kate spent on her Alexander McQueen classic, it's a bargain really.
2 Kate: The highest profile guest apart from the queen, everyone will be watching to see what the new mum will be wearing over her maternity bra and spanx. The last thing she'll want to do is compete with her new sister-in-law so bets are on something dull but classic. At sister Pippa's wedding Kate bled into the background in beige (although a whoppingly expensive €9,000 beige).
3 The Hats: Especially those belonging to the York girls. Beatrice and Eugenie caused much hilarity with their choice of decidedly odd headgear at William and Kate's wedding. Will they opt for an on-trend beret or simple fascinator or splash out on 'look at me' prominence again.
4 Fergie: No longer a member of the royal family and generally shunned at all occasions these days, it seems Harry put in a plea for Sarah Ferguson to be invited. It's thought Prince Philip can't bear her, and she's certainly colourful, but the invitation extends only to the Windsor Castle morning bit and not the 'close family and friends' evening reception at Frogmore, hosted by Prince Charles, who can't abide his former sister-in-law. Will she be on her best behaviour, or be inappropriately scene-stealing by throwing a strop?
5 The Title: Meghan will be awarded a royal title on her marriage and will also be styled Her Royal Highness, as Harry's wife. Formally her name will be Princess Henry of Wales, but only blood princesses are allowed use the title with their first name, so she is never called Princess Meghan (American publications take note). However, a courtesy title will also be awarded to the couple, much like the Cambridges. The 'available' dukedoms are Sussex and Connaught, with betting suspended on the former. HRH The Duchess of Sussex (not used since 1843) is therefore Meghan's most likely moniker from today.
6 The Bridesmaids: Meghan has eschewed a Bridesmaid or American-style Maid of Honour in favour of an assortment of small children, likely to include George and Charlotte. Hilarious potential for tears, tantrums, trips and trauma, but they'll all look cute.
7 The Curtsey: As they depart the altar as man and wife, Meghan will give a customary deep curtsey to the Queen. Her last dip in public was a little wobbly, and she'll be weighed down in a dress and tiara, so it'll be interesting to see if her practicing has paid off. Falling over isn't ruled out.
8 The In-laws: The royals usually tog out well at these things and will be seated according to protocol, always on the right hand side of the church and with no fidgeting or whispering permitted.
9 The Out-laws: Meghan's family is proven tabloid-fodder and the will-they-won't-they shenanigans of her family continued up until the last minute. A lively hodge-podge of ungrateful and estranged half siblings have not been invited, probably for good reason, but mum Doria has been circumspect and under the radar, so Meghan will be hoping it'll all pass off smoothly. A hefty security contingent will ensure there are no last-minute gate-crashers.
10 The Celebs: The Spice Girls are rumoured to be attending, and Sir Elton John has cancelled booked concerts on the day. Tennis champ Serena Williams is a good friend of Meghan along with actress Millie Mackintosh. The Beckhams are virtually guaranteed. No political leaders have been invited, to everyone's relief, avoiding a Downing Street headache over a potential Trump ban.
11 The Kiss: There's no Buckingham Palace balcony scene this year, but Harry and Meghan will be certain to share a smacker on the steps of St George's Chapel, Windsor after the ceremony. Whether it'll be a chaste peck or a lingering Hollywood snog is anyone's guess.
12 The Telly: Some 24 million Britons watched Kate and William's wedding, making it one of the Top 10 most watched programmes ever. There won't be that many this time, but neither beat the 32 million who tuned in to Diana's funeral.