The Princess of Wales was in a design believed to be by Eponine London for tonight’s Royal Carols: Together At Christmas concert. 
The event was at Westminster Abbey and taped for broadcast on Christmas Eve, when it will air on ITV. Yesterday Kensington Palace posted a video of the Princess decorating a Christmas tree in advance of tonight’s concert.
Final touches ahead of the #TogetherAtChristmas Carol Service tomorrow 🎄 pic.twitter.com/mixjI8d5TD
— The Prince and Princess of Wales (@KensingtonRoyal) December 14, 2022
This is the second year for the event. Some of you may remember last year’s concert.
After arriving tonight, the Princess headed inside to greet guests.
It looks like she is speaking with a group of Scouts here. 
The Princess having a laugh as she spoke with guests and performers. 
And saying hello to more youngsters. 
More from this Daily Telegraph story.
The Princess, dressed in an elegant deep red coat dress with matching shoes and clutch bag, greeted clergy at the door of the abbey before going inside, chatting with guests and enthusiastically high-fiving children.
The Princess thanked those taking part in the service, including Hugh Bonneville, the actor, who starred in Paddington, and Melanie C, the singer.
She also spoke to Princesses Eugenie and Beatrice before coming back out to welcome her husband and two children.
Prince William, Prince George, and Princess Charlotte arrived later, and the Princess went outside to meet them. 
We saw a full complement of royal family members at tonight’s concert. Below, King Charles, as he was welcomed by the Dean of Westminster, David Hoyle. 
Camilla, Queen Consort. 
Other royal family members attending included Sophie, the Countess of Wessex. 
Princess Beatrice & her husband, Edoardo Mapelli Mozzi. 
Princess Eugenie & her husband, Jack Brooksbank. 
Zara and Mike Tindall.
Princess Alexandra. 
The Duke of Kent. 
The Duchess of Gloucester. 
The Duke of Gloucester. 
Prince and Princess Michael of Kent. 
The Kents’ daughter, Lady Gabriella Kingston. 
Below, the Kents’ son and daughter-in-law, Lord and Lady Frederick Windsor (née Sophie Winkleman). 
Mike and Carole Middleton, Pippa and James Matthews. 
Below, the Wales family making its way to their seats. 
A look at some of the decorations inside Westminster Abbey. 
More from The Telegraph piece.
A Christmas tree in the abbey was decorated with small Paddington Bear decorations, a nod to the late Queen’s famous sketch, while guests were greeted with atmospheric snowflakes from a snow machine as they arrived at the entrance.
There were about 1800 people at tonight’s concert, most nominated by … “Lord Lieutenancies across the UK, alongside community networks and charities associated with Members of the Royal Family, including some of the patronages of Her Late Majesty. The congregation will also consist of those who may be more vulnerable or isolated in our communities – including people who might find the winter period particularly challenging,” according to a news release. 
Tonight’s service “will recognize the selfless efforts of individuals, families, and communities across the UK, and celebrate and showcase the joy that human connection and togetherness can bring,” per a Kensington Palace news release. More from that release:
This year’s carol service is dedicated to Her Majesty Queen Elizabeth II and the values Her Majesty demonstrated throughout her life, including duty, empathy, faith, service, kindness, compassion and support for others. These principles are shared and personified by the inspirational guests who have been invited to the Abbey in recognition of their tireless work to help and care for those around them.
Among the guests, little Theo Crompton.
Theo is the charming boy who was so excited to present the Princess with flowers in September during a visit to Wales.
Theo’s face when meeting the Princess in September. 
After the taping, the Kensington Palace social media accounts shared a photo and message for Theo. 
A look at the Order of Service for tonight’s concert, with artwork created by artist Aurelie Baudry Palmer. 
More on the special broadcast from The Mirror’s coverage.
There will be performances from Craig David, Alexis French and Samantha Barks, and a special duet from Alfie Boe and former Spice Girl Melanie C.
Readings from Prince William, Dame Kristin Scott Thomas, Hugh Bonneville and Kadeena Cox have also been organised.
The Choir of Westminster Abbey sang many of tonight’s selections, and the St. Mary’s Ukrainian Choir led the singing of “Away in a manger.” Another carol sung this evening was “O come, All Ye Faithful.” More on that element of the program from this People article.
Last month, Kate asked for the public’s help in deciding the concert’s finale, sharing a poll between three songs: “Hark! The Herald,” “Joy To The World” and “O Come, All Ye Faithful.” The winner was “O Come, All Ye Faithful,” with 40.7% of the vote after nearly 25,000 people made their pick.
In addition to the Westminster Abbey concert, eight more Together at Christmas services are being held, including events in Belfast, Birmingham, and Swansea. More about those events from a Kensington Palace news release.
….a family of ‘Together at Christmas’ community carol services will be taking place around the UK across December to celebrate local heroes. Hosted by Lord Lieutenants, and supported by The Royal Foundation, these events will be tailored to their local communities whilst also including elements of the Westminster Abbey service. Guests will also receive a special foreword written by The Princess of Wales.
A look at the Wales family as they prepared to leave after the taping. 
The King and Queen Consort as they left the Abbey tonight. 
And the Wales family as they departed. You can see how Princess Charlotte’s outfit matched her mother’s ensemble, and Prince George’s look was similar to his father’s. 
Now for our look at what Kate wore for today’s concert and in the quick video when she is decorating the tree, starting with tonight’s ensemble. 
The Princess appeared to be wearing a version of the Eponine London Chi Chi Coatdress worn at a November 2021 COP26 reception (below right). The dress is midi-length and features a vee-neckline, a ‘wrap-style’ front, pleating details at the neckline, a fitted waist, and side belting with decorative buttons. Tonight’s version has long sleeves with less gathering at the shoulder and waist. 
Middleton Maven and Carly at Kate Middleton Style both thought this was a version of the Chi Chi coatdress. Carly contacted the label to ask if the Princess was wearing one of their designs, and they responded with a smiley face, so I think it’s likely an Eponine piece.
The Princess carried a new bag that looked much like Aspinal’s Mayfair Clutch ($330 on sale, regularly $445), but in close-ups, I don’t see the signature turn-lock on the front, so I am still looking at handbags. Tonight’s gloves look much like the Bow Detail Glove by M and Co. gloves carried for Christmas morning church at Sandringham in 2018. 
The Gianvito Rossi 105 pumps were back.
This evening the Princess wore a new pair (at least to us) of Accessorize earrings, the Eclectic Gem Statement Long Drop Earrings (£7 on sale). The style features “…rich vibrant berry tones… with a collection of eclectic stones.” Thank you to Kate Middleton Style and UFO No More for their speedy earring ID. 
In the video, the Princess wore a Holland Cooper sweater. 
We first saw the Duchess of Cambridge wearing the sweater in a video for Children’s Mental Health Week in February. 
It is the Holland Cooper Fairisle Knit Sweater in cream (£179, roughly $220 at today’s exchange rates). The piece is described as being made from an “extra-soft yarn,” a blend of nylon, alpaca, and wool. 
It has ribbing at the extended cuffs, hem, rolled neck, and lots of Holland Cooper buttons at the cuff and shoulder. The sweater is currently sold out, but you can sign up to be notified by email when it is back.
The trousers worn in the video appear to be the Roland Mouret Axon style worn on several occasions. 
UPDATE WEDNESDAY, DEC. 21: The Princess appears to be wearing new earrings in the video that were also worn for a tree-planting ceremony at Westminster Abbey on December 14, shown below left.
VIDEOS:
Westminster Abbey tweeted a video as the Wales family left after tonight’s concert.
Earlier tonight TRH The Prince and Princess of Wales left the carol service at Westminster Abbey with Prince George and Princess Charlotte. #RoyalCarols#TogetherAtChristmas pic.twitter.com/XUTrhQ76Io
— Westminster Abbey (@wabbey) December 15, 2022
This is the full video as the Duchess of Cambridge accompanied singer/songwriter Tom Walker at last year’s concert when he performed his Christmas single, “For Those Who Can’t Be Here.”
The Royal Family Channel offers more than three minutes coverage of tonight’s engagement.
Kay
Wednesday 21st of December 2022
I would imagine a great deal of planning go into most outfit choices and one consideration would be how it photographs and the timeless nature when the photos are still being viewed decade's later. Can completely understand why often the outfits are conservative so not to detract from the event. Would be horrid if Katherine was an absolute slave to the current fashion of the time..would be some tad hideous pieces that when we reflect should never see the light of day, again. Not that Kate ever wears them. Kates outfit today, simply sublime.
martha
Tuesday 20th of December 2022
simply stunning. Clean lines, beautiful tailoring, elegant and sleek with minimal adornment eg bling. The gorgeous heels do the outfit justice, any other shoe style would have made the outfit look clunky, heavy and dowdy.
The colour is burgundy – or as fashionistas prefer ‘viva magenta’ – has been hailed by the fashion colour bible, Pantone, as the colour of 2023. It is described as ‘audacious, full of wit, and inclusive of all style and elegance.
Catherine is really increasing the fashion style and elegance. Sophisticated and elegant.
vava
Tuesday 20th of December 2022
I prefer the red coat from last year's event over this one. The burgundy is a nice rich color, but for this event it seems a bit drab to me. Maybe it does need some flashy jewelry. I'm a shoe person and would love it if she'd change out her shoe game. These four inch stilettos, while classy, just bore me to death.
Larissa
Monday 19th of December 2022
Given that so many of Kate's coats are very similar in style, I feel like I should welcome something a bit different – but, unfortunately, I'm rather bored by this coat. I can't seem to place my finger on exactly what I don't like about the fit; something about the wrap front and the placement of the waist seem off to me and make the coat less flattering than it could be. The only decorative element, the buttons, are barely noticeable without a contrasting colour. And, once again, the length would be so much better if it was just a few inches shorter.
The earrings are an interesting choice to me. I remember when Kate wore Accessorize earrings at the beginning of year, the generally accepted explanation was that they were a Christmas gift from the children. I wonder if the same could be said for these – did the kids contribute to their mum's outfit for her big event? Is the low price point an acknowledgement of the cost of living crisis in the UK? I have to admit those earrings immediately caught my eye, and I was very surprised to find them so affordable. They're a lovely pair and don't look cheap when photographed.
Shirley
Monday 19th of December 2022
Stunning!!!