Ingrid Seward on Princess Diana’s Death: “There Are Very Few Moments Like Those in History”

If you are old enough to remember what you were doing on August 31, 1997 and have even a passing interest in the royal family or pop culture, you likely remember every detail of the moment you found out that Princess Diana had been injured in a car accident in Paris (as was initially reported) and later reports that she had been killed. (This writer remembers everything.) Though very few of us actually knew the Princess of Wales, the most famous woman in the world was a part of our cultural lexicon and fabric, a fixture on magazine covers and news programs worldwide. And, suddenly, inexplicably, at just 36 years old, she was gone.

As PEOPLE succinctly puts it, “Princess Diana’s tragic death united the world in grief.” Almost unbelievably, a quarter-century has passed since that awful day in August: this Wednesday marks 25 years since the accident, which also claimed the lives of Diana’s companion Dodi Al-Fayed and driver Henri Paul. The Queen’s biographer Ingrid Seward—who knew Diana personally—tells PEOPLE “I remember where I was in every tiny detail when I heard about Diana,” she says. “It affected everybody. Almost everyone in the world remembers where they were that day—it was an extraordinary, global phenomenon.”

Mourners flocked in droves to Diana’s London home of Kensington Palace, as well as other royal residences like Buckingham Palace and St. James’s Palace, to lay flowers, notes, and photos expressing their sorrow at her loss.

“It was such a shock that it remains imprinted on your mind,” Seward says. “There are very few moments like those in history.”

Ed Perkins, who directed the recently released documentary The Princess about Diana, says “this is a story of someone who managed to effect change in people’s lives, and continues to do so. You had someone who people could project their own hopes and dreams and fears onto and find a strangely personal connection to. It was only once she was gone that lots of people realized what they’d lost.”

Diana lives on through her sons, Prince William and Prince Harry, who were 15 and 12 at the time of the crash and are now 40 and 37. In the documentary Diana, Our Mother, Harry said “it was very, very strange after her death, the sort of outpouring of love and emotion from so many people that had never even met her.”

Diana’s former bodyguard Ken Wharfe calls William “the living example of his mother”; Harry has called himself “my mother’s son,” saying that he looks to honor Diana in everything he does. And, in addition to the events of August 31 leaving a searing impression on the mind, who can forget the heartbreak of watching the boys walk behind their mother’s casket at her funeral the next weekend, and the childlike scrawl of “Mummy”—written by Harry—on a card atop a floral arrangement?

Twenty-five years later, and that image still hurts.