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It was just after 9 a.m. on September 8, and the weather in New York City was perfect. The sun was shining. The temperature hovered at around 70 degrees. The dew points were low. And on Bleecker Street, it seemed like every woman on their way to get coffee or to the gym was wearing the exact same thing: bike shorts and a baggy sweatshirt. Including me.

Sometimes I can’t tell you why I succumbed to a clothing trend—so much of what hangs in my closet feels like it was purchased by Adam Smith’s invisible Essie-polished invisible hand. But this particular outfit? I can tell you exactly where it came from: Princess Diana.

Oh, yes. Every September, users on TikTok, Reddit, and Instagram begin to recirculate old photos of Princess Diana leaving the gym in spandex and a statement pullover. A popular image shows her wearing a sweatshirt with the Virgin Atlantic logo; another, with the emblem of Harvard. (She had a thing for American universities: in addition to the Ivy League school, she often wore a bright purple design from Northwestern.) Some have even taken to calling it Princess Diana Fall—or, those glorious few weeks in September where it is warm enough to still show off your legs but just cold enough to wear a comfy sweatshirt.

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Princess Diana after leaving a gym in Earl’s Court.Photo: Getty Images

How did it catch on? Sure, part of the reason is that Diana was one of the best-dressed women in history and therefore a perennial muse to designers. (Tory Burch, for example, cited her as an inspiration for her spring 2020 collection, while Rowing Blazers recently re-released her Warm & Wonderful sweater.) Yet, her athleisure looks also have a fascinating history.

Princess Diana never liked being photographed in her exercise clothes—and even took great lengths to avoid doing so. Yet there was an incredible demand for private images of the Princess: in 1993, The Mirror paid a reported 150,000 pounds for photos of Diana working out at her health club. (The Princess later sought legal action.) In the parking lot of her gym, she’d often walk backwards from her car to avoid showing her face to photographers. But her greatest hack? She’d wear the same outfit every time: spandex bike shorts with a sweatshirt, again and again. By doing so, it would decrease the value of the photograph taken by paparazzi—no newspaper would pay a premium to run an outfit in their pages they’d already run before.

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Princess Diana leaving Chelsea Harbour Club in 1995.Photo: Getty Images

“Every single session all the media were outside camped with their stepladders, and cameras and lenses and everything,” her personal trainer Jenni Rivett recalled in a 2018 interview. “ It wasn’t her that asked for all this. I remember one of her strategies was that she was going to wear the same Virgin Active sweatshirt every single session.”

A side effect of her dedication to sartorial repetition is that the style became synonymous with Diana herself. Plus, whereas much of what Princess Diana wore was custom or designer, this outfit was decidedly—and achievably—high street. “It’s not the combination itself that is novel—that’s what people worked out in at the time,” says Vogue’s senior runway editor Laia Garcia-Furtado. “But on her, it becomes transformed, perhaps because it was so rare to see someone of her stature ‘dressed down’—and also because it is endowed by her ineffable allure.”

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Princess Diana leaving Chelsea Harbour Club in 1994.Photo: Getty Images
During the 2010s, internet and social media culture reached its full power. Pictures of Diana weren’t just in magazines or newspapers, but all over blogs, Twitter feeds, and Instagram Discover pages. It was also around this time that athleisure became a major trend, with companies like Lululemon and Alo Yoga hitting billion-dollar revenues. Suddenly, Diana’s ’90s gym clothes felt downright fashionable. And in 2019, it all hit a fever pitch when Hailey Bieber—an extremely influential fashion figure in her own right—recreated Diana’s bike shorts and sweatshirt look for a Vogue France editorial. The images went viral.

For the next few weeks, the weather in the large swaths of the United States will be the most pleasant of the year. So go ahead, throw on your favorite sweatshirt, and head out the door. It would be rather regal of you.