Princess Diana in August 1997.Photo: Kent Gavin/getty

HRH, The Princess of Wales, Princess Diana, on her arrival at Sarajevo airport Friday, 8th August 1997 at the start of her two-day tour to publicise the international problem of land mines. . Picture taken 8th August 1997. (Photo by Kent Gavin/Mirrorpix/Getty Images)

Princess Diana’s tragic death united the world in grief.

Diana was just 36 when she was killed in a Paris car crash, which also took the lives of Dodi Al Fayed and driver Henri Paul, on Aug. 31, 1997.

“It was such a shock that it remains imprinted on your mind,“Queen Elizabethbiographer Ingrid Seward, who knew Diana personally, tells PEOPLE in this week’s cover story. “There are very few moments like those in history.”

“I remember where I was in every tiny detail when I heard about Diana,” Steward adds. “It affected everybody. Almost everyone in the world remembers where they were that day — it was an extraordinary, global phenomenon.”

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Mourners flocked to Diana’s London home of Kensington Palace, Buckingham Palace, St. James’s Palace and other locations to lay flowers, notes and photos expressing their sorrow.

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“This is a story of someone who managed to effect change in people’s lives. And continues to do so,” says Ed Perkins, director of the new HBO documentaryThe Princess. “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.”

Flowers outside Kensington Palace after Princess Diana’s death.Liba Taylor/CORBIS/Corbis via Getty

A multi-colored sea of floral tributes to Diana, Princess of Wales, lie outside the gates of her London home. The flowers began to arrive soon after news of Diana’s death, in a Paris car crash, reached Britain.

Princess Diana’s two sons,Prince WilliamandPrince Harry, were just 15 and 12 respectively at the time of her death.

In the documentaryDiana, Our Mother,Prince Harryrecalled, “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.”

From left: Prince Philip, Prince William, Charles Spencer, Prince Harry and Prince Charles walk behind Princess Diana’s casket at her 1997 funeral.JEFF J. MITCHELL/AFP/Getty

Prince Charles

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Both William and Harry have kept their mother’s legacy alive throughout their lives.Prince Williamcontinues to support causes close to his mother’s heart and mirrors his mom’s hands-on approach to parentingPrince George, 9,Princess Charlotte, 7, andPrince Louis, 4. His recent decision to move his family from London to the countryside of Windsor reflects Diana’s wishes for him to have as normal an upbringing as possible.

“William is the living example of his mother,” says Diana’s former bodyguard Ken Wharfe.

Meanwhile,Prince Harryhas called himself “my mother’s son,” saying that he tries to honor Diana in everything he does.

“Harry has made it perfectly clear that every decision he has made he refers upstairs to his mother,” says Andrew Morton, Princess Diana’s biographer. “His decision to go to America — he felt she was watching over him and giving him the thumbs-up.”

source: people.com