WATCH: Go Inside Queen Elizabeth's Incredible Closet

See a sneak preview of a new fashion exhibit at Buckingham Palace

Photo: DANIEL LEAL-OLIVAS/AFP/Getty

Queen Elizabeth is opening her closet doors.

Showcasing 10 decades of outfits – from the Queen’s own lace christening gown worn in 1926 to the “neon at 90” look worn at Trooping the Colour in June, Fashioning a Reign: 90 Years of Style from the Queen’s Wardrobe, which officially opens July 23, is the biggest collection of the monarch’s fashion and millinery to ever go on public display.

“It’s a unique and unprecedented display of fashion dress from Her Majesty the Queen’s wardrobe,” Caroline de Guitaut, curator of the much-anticipated exhibition at Buckingham Palace, tells PEOPLE, adding that many of the outfits have never been on public display before.

That includes the Queen’s wedding and coronation dresses, which are displayed together for the first time. Both designed by British couturier Norman Hartnell, and worn in 1947 and 1953 respectively, the pieces are described by Guitaut as a “tour de force of craftsmanship.” Hartnell sent nine sketches of the coronation gown to the Queen before she approved the final design. (Among her requests: More color in the intricate embroidery.)

For many more secrets from inside the Queen’s closet, pick up this week’s issue of PEOPLE, on newsstands Friday

With 75 outfits on display and more than 80 hats, other highlights include the dresses, robes and coronets worn by then-Princess Elizabeth for the 1937 coronation of her parents, King George VI and Queen Elizabeth, as well as more modern ensembles such as the Stewart Parvin neon-green outfit worn during her 90th birthday celebrations.

The exhibition, which is part of the yearly summer opening of the State Rooms at Buckingham Palace, has been described by the Royal Collection Trust as their “most ambitious” yet and is expected to play host to more than half a million spectators during its 10-week run.

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One VIP spectator who has already been given a sneak preview? The Queen herself.

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“She has seen it,” reveals Guitaut, adding that the monarch appeared to approve. “I think she found it interesting to see all the things again, to see the film footage and she was very interested in the way it was all set up and seemed very engaged – she seemed to enjoy it.”

On whether she had a favorite piece, the curator revealed it was the two most sentimental garments that seemed to capture the Queen’s attention the most. “I think she always likes to see her wedding dress and her coronation dress – those are very meaningful.”

Fashioning a Reign: 90 Years of Style from the Queen’s Wardrobe is at Buckingham Palace from July 23 until October 2. It follows the debut exhibit at the Palace of Holyroodhouse, which opened on April 21 and continues until October 16. The final installment of the trio (where are all different in content and theme) opens at Windsor Castle on September 17 and finishes on January 8, 2017.