The Duchess wore a new Catherine Walker coatdress for today’s National Service of Thanksgiving commemorating the Queen’s 90th birthday.
Kate, William and Harry arriving for the service this morning, held at St. Paul’s Cathedral.
Her Majesty on the way into the Cathedral with the Duke of Edinburgh. Today is also his 95th birthday.
The guest of honor was actually a little late. More from The Telegraph.
Apparently the Queen was delayed by 10 minutes because of a traffic accident at Embankment, but as the crowds caught sight of the vehicle they cheered.
A fanfare sounds as HM and The Duke arrive at the National Service of Thanksgiving #Queenat90 @StPaulsLondonhttps://t.co/qyNbKJgYnh
— The Royal Family (@RoyalFamily) June 10, 2016
I believe the railing seen in photos of the St. Paul’s steps is temporary, installed for today’s event. 
St. Paul’s captioned this photo “Members of the congregation delighted by the Queen’s arrival for the National Service of Thanksgiving” on the Cathedral Facebook page. HM wore a primrose yellow coatdress with a matching hat, both by Angela Kelly.
More about today’s service from The Mirror:
A total of 53 members of the extended Royal Family packed into the 18th century Cathedral for the service, which was also attended by dignitaries from around the world and Brits representing public service.
From left to right: Prince Andrew, Princess Eugenie, Prince Edward, Princess Beatrice, James-Viscount Severn, Prince Harry, Lady Louise Windsor, the Duchess of Cambridge, Sophie-Countess of Wessex, Prince William. 
Kate curtseys as Harry and William bow when the Queen leaves the Cathedral. 
Following the service, there was a Buckingham Palace reception for visiting Governors-General from Commonwealth Realms. 
Those royals not attending the Palace gathering took part in a reception at the Guildhall.
You can see Sophie, the Countess of Wessex, and Prince Edward in this shot.
Edward and Sophie’s daughter, Lady Louise Windsor, is wearing pink Zara and a Jane Taylor headband, according to Anna the HRH Countess of Wessex blog; below center, Prince and Princess Michael of Kent; Prince Harry is on the right. All are shown as they arrived at the Guildhall.
All Photos by Rebecca English, The Daily Mail & Mail Online (@RE_DailyMail)
There has been quite a bit of chatter about Lady Amelia Windsor’s look for today’s service. She wore a Chanel piece that blended the brand’s updated tweed with pleating on the front of the dress, as well as the belled bracelet-length sleeves. (I believe Lady Amelia has interned at Chanel.) Her grandfather is the Queen’s cousin, Prince Edward, Duke of Kent. Lady Amelia made her Paris debut in 2013, she is 20 years old. 
Lord Freddie Windsor is the only son of Prince and Princess Michael of Kent; Lady Gabriella is his sister. 
Now to what Kate wore, a wool crepe coatdress by Catherine Walker. 
In this video, you can see how Kate’s coat moves.
The Duke and Duchess of Cambridge & Prince Harry arrive at the Guildhall #Queenat90 pic.twitter.com/p0EUBoPGwq — Aine Fox (@aine_fox) June 10, 2016
Here are three more views of the Duchess. 
The garment combines elements from multiple Catherine Walker designs. Below left, the Rosa Coatdress; on the right, the Melrose Coatdress.
Some thought the piece reminiscent of a Catherine Walker style worn by Carole Middleton. Below we show the two pieces probably prompting that reaction; on the left, the ensemble seen at Kate and William’s wedding, and on the right, the look worn for Charlotte’s christening.
Another view of Kate at St. Paul’s today.
And a closeup of the lace. 
The Duchess accessorized with what has become her go-to bag, the Nina Clutch in trench by LK Bennett.
She wore the Gianvito Rossi heels we first saw at the Buckingham Palace Garden Party in May.
UPDATE JUNE 15: Kate wears Gianvito Rossi suede heels in the ‘Praline’ colorway. It turns out that ‘praline’ is the correct color name, with thanks to Tiffany C. and Elizabeth of Kate’s Closet, they learned that info from Gianvito Rossi. We show the shoe at My Theresa, where it seems to be available in all sizes ($675). It is also offered at Net-a-Porter where the color is called ‘antique rose,’ and it is available in most sizes ($675).
Kate’s hat is the Jane Taylor style first seen for Princess Charlotte’s christening.
A closer look at the hat and Kate’s updo.
For jewelry the Duchess wore a new pendant paired with the aqua/diamond drop earrings we have seen previously.
On a side note, The Daily Mail’s Rebecca English shared via Twitter a fun Charlotte update.
Rebecca English, The Daily Mail & Mail Online (@RE_DailyMail)
It is possible we’ll see Charlotte tomorrow, along with big brother George, at Trooping the Colour. One more look at Kate as she leaves St. Paul’s Cathedral.
And another shot of the Queen and Prince Philip, with all best birthday wishes to both.
The service concluded with God Save the Queen.
Congregation at St Paul’s Cathedral sings God Save the Queen #Queenat90https://t.co/MClJlPeFPB https://t.co/GZcKHYeda1
— BBC News (UK) (@BBCNews) June 10, 2016
We leave you with ’90 Photos for 90 Years’ posted on Twitter by The Royal Family.
.@StPaulsLondon service celebrates a special milestone in The Queen’s life. Here’s 90 photos for 90 years #Queenat90https://t.co/RUQhUbKKyN — The Royal Family (@RoyalFamily) June 10, 2016
LINKAGE:
- The Daily Telegraph’s live blog is here and photo gallery here; The Mirror’s story about the service is here; The Daily Mail’s photo-laden story is here, The Express photo gallery is here,
Pamela S
Sunday 20th of May 2018
This pale blue with the white lace is one of my absolute favorites. It has the proper shape, the movement is lovely, the neckline has a more open style instead of that choker look she wears far too often. It's sleek, young, stylish, and a great color for her.
I have to agree with everyone on the shoes; that color just doesn't do it for me with this outfit. as far as the "no suede in summer", that must be a southern US guideline. I was raised in Michigan and I've never heard that.
Emma
Wednesday 15th of June 2016
I'm sure Kate's wearing the "Blush" shade!
Sarah
Monday 13th of June 2016
I love the hat with the coat dress and think it is elegant. The shoes seem to be off and I am not sure if they are photographing wrong or just wrong. It seems a different shoe could have really made this look pop. Of course maybe she was opting for a pair that she has worn in rather then a new shoe to break in.
katefanatic
Monday 13th of June 2016
First the good---hair, makeup, jewelry, shoes and clutch---perfect. Also, the neckline on the coat was simply gorgeous and I was so happy to not have that "buttoned up to the throat" look. Hemline was exactly where it needed to be and again happy not to see a midi length. Tailoring on the coat was superb.
Now the not so good---I think that she needed to lose either the frou frou hat or the lace on the front of the dress. IMO either would have worked fine but not both. Hat but no lace or lace but a much simpler hat---either would have been fantastic. The two together served to make this look a tad matronly.
I don't get the criticism of the shoes. I also do not get the love for lady Amelia's Chanel ensemble---it didn't fit correctly across the bust and looked funeral to me.
Arlene
Sunday 12th of June 2016
Kate looks lovely. Her wardrobe choices will stand the test of time.