The Princess of Wales chose designs by Alexander McQueen and Philip Treacy for Royal Ascot.
It was Day Four of the annual racing event. Below, the King and Queen as they arrived in their carriage. 
There was a full royal roster at the racecourse today. 
Princess Michael of Kent and the Duchess of Edinburgh.
Dame Judi Dench and her partner, David Mills.
Another view of the Prince and Princess of Wales as well as Princess Beatrice and Edoardo Mapelli Mozzi.
Here is a wide shot. 
A wave from the Princess. 
The Princess of Wales and Princess Beatrice. 
In this photo, you see the Princess with Melissa Baker. Oliver Baker is a longtime friend of the Prince and Princess and a godfather to Prince George.
Here you see the Princess and Queen Camilla chatting.
In addition to watching the races, the King and Queen visited an exhibit at the racecourse celebrating Her Late Majesty Queen Elizabeth II’s time at Royal Ascot.
In the first #RoyalAscot since the passing of Queen Elizabeth II, King Charles III and Queen Camilla visit a photographic exhibition paying tribute to Her Late Majesty at the Royal Meeting @Ascot… pic.twitter.com/xoCRH0Ry09
— At The Races (@AtTheRaces) June 23, 2023
The main event, of course, is the racing. Below, Princess Beatrice and her husband.
Prince Michael of Kent, one of the late Queen’s cousins, and his wife, Princess Michael. 
The Princess of Wales and Dame Judi Dench. 
The Duchess of Edinburgh and Princess of Wales as they watched today’s racing. 
More from the Daily Mail’s story.
The Princess and her royal relative, Sophie, Duchess of Edinburgh, shared sweet moments as they laughed together while watching the action unfold on the course – while Kate was spotted giving the Duchess a thumbs up as she celebrated a win on the horse she had backed.
Sophie appeared to place a bet on Porta Fortuna, who registered an impressive success in the Albany Stakes to provide Frankie Dettori with his 80th Royal Ascot win.
The Duchess celebrating. 
And a thumbs-up from the Princess. 
The Prince and Princess of Wales had official duties today. 
Below, you see them as they presented the winner of the Commonwealth Cup with a trophy. Today’s winner was “Shaquille,” ridden by Oisin Murphy. 
The Princess as she shook hands with the jockey. 
A quick recap of royal racing styles begins with the Queen, who wore an Anna Valentine design and a Philip Treacy hat.
The Duchess of Edinburgh sported the ‘Fontaine’ Silk Crepe Dress ($3200) by Suzannah London and a hat by Jane Taylor. 
Princess Beatrice was in a dress by Monique Lhuillier, with thanks to Laura for the ID, and a hat by Justine Bradley-Hill. 
Thank you also to Polka Dot Popp and Royal Fashion Police for ID information.
Now for our look at what Kate wore for Royal Ascot. 
She was in a rich red dress by Alexander McQueen that featured a v-neck, wrap-effect bodice, raised, puffed shoulders, defined waist, and a full a-line skirt.
UPDATE JUNE 24: With thanks to Royal Fashion Police on Twitter, and others, it looks like this could be the dress worn by the Princess when visiting the Welsh Guards on St. David’s Day.
Today’s look was a departure from what we have seen in previous Royal Ascot appearances. Thoughts on the look come via this column by Bethan Holt, the Telegraph’s fashion director.
For all her previous appearances at Royal Ascot, the Princess of Wales has worn pretty white or pastel dresses. But that was then – this year she’s in attendance for the first time in her newly elevated position and it was, evidently, time for a change. Out with the soft and feminine, in with the pop of power colour.
She may have switched up her colour palette, but the overall feel of the Princess’s outfit was elegant, timeless and feminine; with its giant floral decoration, her hat had a sophisticated vintage effect while her dress came with McQueen’s signature tailored flourishes – like puffed sleeves, a nipped waist and carefully sculpted neckline – but with a swishy pleated skirt which stopped the look feeling too serious.
Here are the previous looks worn for Royal Ascot: the lace Dolce & Gabbana piece worn for Kate’s Royal Ascot debut in 2016, the lacy white Alexander McQueen seen at Royal Ascot 2017, the blue Elie Saab separates worn in 2019, and last year’s cream and brown Alessandra Rich design. 
We saw another hat by Philip Treacy, whose designs the Princess has worn for her last three engagements requiring a hat. 
The Princess wore a new pair of red suede heels, the Jennifer Chamandi’ Lorenzo’ 105 style (£580, about $730). 
The product description reads, “The Lorenzo can be worn two ways: with or without a fine leather strap threaded through the Eye of the Needle and buckled across the foot, ensuring you get maximum mileage from your beautiful new shoes. The strap is removable for versatility, offering both a more classic and more trend-forward look.” Thank you to Middleton Maven for her speedy shoe ID.
The Princess carried a vintage Hermès clutch, the Rio in rouge leather. This is the first time we have seen the Princess with Hermès, very much a favorite of the late Queen, and also King Charles. 
The bag measures approximately 9″ x 6″ and features the brand’s signature H logo atop a button snap closure. These photos are from a 1st Dibs listing (now sold), and the product description notes the bag carries a 1992 date stamp. Thank you to Mya and Laura for their ID work on this item!
Today’s earrings are a past-season design by Sézane, the Esme style. 
Here is one more look from today. 
I thought we might see the Princess with a bag or perhaps shoes by LK Bennett; this year the retailer is the event’s official womenswear sponsor. 
LK Bennett created a Royal Ascot collection featuring dresses, shoes, handbags, and more. More from the Ascot site.
As the Official Womenswear Sponsor of Ascot and Royal Ascot, LK Bennett has created an elegant capsule collection of clothing, shoes and accessories all designed with Ascot in mind.You’ll discover beautiful pieces that adhere to the famous Royal Ascot dress code with signature court shoes and silk scarves to match.
VIDEOS:
Here is a video of the carriage ride.
The Prince and Princess of Wales arrive in the Royal Procession at #RoyalAscot! pic.twitter.com/1RcAW3Mc7t
— At The Races (@AtTheRaces) June 23, 2023
And another video showing the trophy presentation.
Prince William and Princess Kate make the presentation to the winning connections of Commonwealth Cup winner Shaquille! #RoyalAscot pic.twitter.com/Z2el1A6N7R
— At The Races (@AtTheRaces) June 23, 2023
Joan
Monday 3rd of July 2023
This style is lovely and—if you squint—the cut is quite similar to her matron-of-honor dress worn for Pippa’s wedding.
Jack Bing
Thursday 29th of June 2023
At first glance I thought this was a reworked version of the Beulah evening gown worn in 2011 for the 100 Women in Hedge Funds event because of the colour and sleeve shape. But the neckline and texture of the fabric are different. I would be interested to see evening dresses turned into day dresses and vice versa; creativity and re-wearing combined!
Emily
Tuesday 27th of June 2023
I really like this outfit, including the earrings, which normally I wouldn't have gone for! but I think they work with the brightness (almost tropical?) feel of the dress and the hat, and I feel like her hairdo screams out for big earrings like these. So summery and pretty.
Zell
Tuesday 27th of June 2023
@Emily, I agree, the bright, intense red does remind one of the tropical beauty of vibrant flowers, with an ultra bright Summer feeling the earrings enhance. Great description!
Kiki Vancouver
Tuesday 27th of June 2023
That hat is absolutely fabulous, but the earrings don’t go with it at all - very strange and jarring pairing🤔 Beatrice looks great too - her style game keeps going from strength to strength.
Sansa Stark
Monday 26th of June 2023
That bag! J'adore!