The first day of fall may inspire a desire to stock up on pumpkin spice, but as soon as the leaves begin to change color, royal fans will experience another phenomenon. Yes, Princess Diana's season has arrived.

Here on the East Coast, we experience the typical wild swings of early fall temperatures, and when it's 80 degrees one day and 68 the next, it's hard to know what to wear. Diana's casual uniform is bike shorts and a sweatshirt.

The late Princess of Wales was often photographed heading to the gym in lycra shorts and a crew-neck graphic sweatshirt, wearing everything from the iconic navy Virgin Atlantic design to the gray Harvard style.

While she preferred to wear coral shorts, Princess also went to the gym and ran errands in shades of black, white, and navy.

And of course, the key to the ultimate '90s Diana look is a pair of chunky white sneakers and slouchy white socks.

Even Cardi B. is not immune to the gravitational pull of a sweatshirt and biker shorts.

I happen to be wearing such a combination right now, but my sweatshirt says “Lucerne, Switzerland” on it, a hasty purchase from the gift store this summer. But no matter where your athleisure is from, you're sure to get a royal vibe from this look.

Sadly, Princess Diana's unofficial uniform will only be worn for a short time. And winter will be here before we know it, but I'll be enjoying every minute of these fall days (in my Virgin Atlantic Airways sweatshirt, of course).