Princess Eugenie Glows in Never-Before-Seen Photo Shared by Reception Gown Designer Zac Posen

Birthday girl Princess Eugenie is a beauty in blush!

After tying the knot in St. George’s Chapel at Windsor Castle in October wearing a custom long-sleeved, open-back ivory gown from the British-based design house Peter Pilotto, Eugenie changed into a gorgeous blush silk gown from American designer Zac Posen for her private evening reception at Royal Lodge. In honor of the royal’s 29th birthday on Saturday, Posen shared a behind-the-scenes photo of Eugenie trying on the incredible dress.

“Every now and then you meet somebody who just knocks your socks off,” the designer captioned the never-before-seen snap. “This last year I got to know a young lady who possessed natural elegance, a heart of gold and sophisticated humor. My studio and I had the honor (and the best time) collaborating with her on her special gown. Best of all she is dedicated to using her privilege to help our world and its people with her project @the_anti_slavery_collective.”

He concluded the sweet message, “Happy Birthday! @princesseugenie.”

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Posen previously said he was inspired by the beauty of Windsor and the surrounding countryside for the royal bride’s ethereal second look.

The color reflects the blush of an English rose, and Posen says he took inspiration from the White Rose of York. In fact, a White Rose of York is subtlety embroidered on both the shoulder and back of the dress, which hold the cape detail together.

The plissé silk fabric, which is cut into signature drapes at the bottom of the gown, comes from the British Mill, Biddle Sawyer Silk.

Princess Eugenie talked about working with Posen on her reception gown for the current exhibition of her royal wedding ensembles at Windsor Castle.

“I’ve had a lifelong admiration for him as a designer – and because Zac is based in New York and I lived there for some time when I was younger, and for me it was an easy decision to make and Zac is such a cool, fun, brilliant designer and friend,” Eugenie shares on an audio guide. “I wanted something reminiscent of Grace Kelly in ‘To Catch a Thief,’ so I showed that for reference and Zac came up with this silk that he had found from Manchester, every single draping effect, every single detail, every button is all painstakingly done by him and his team.”

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Princess used a photo wearing the blush stunner at her wedding reception with husband Jack Brooksbank to thank fans for their well-wishes on her birthday.

“My 28th year was the best one yet,” she wrote on Instagram. “Now to being 29…Thank you for all the birthday wishes.”

Posen wasn’t alone in his praise for the birthday girl. Both of Eugenie’s parents, Sarah Ferguson (affectionately known as “Fergie”) and Prince Andrew, dedicated social media posts to their youngest daughter on her special day.

Prince Andrew shared a collage of photos going back to Eugenie’s childhood, including a family portrait featuring Eugenie as a baby (and big sister Princess Beatrice adorably enthralled by her new sibling) and a photo of the Queen’s granddaughter riding a horse with her father close by. He also included a shot of the duo at her wedding and a photo of Fergie and her two daughters.

“Wishing @princesseugenie a very Happy Birthday!” he captioned the post, adding the hashtag #HappyBirthdayHRH 🎉.

Fergie included the wedding reception photo and a throwback photo of her daughter as a little girl.

“Happy Birthday my darling girl xxx,” she wrote.