In the aftermath, memorials to the fallen princess have abounded. One of the most prominent is her family home, Althorp, where her body is interred.
Diana's family came to Althorp as sheep farmers, building a house there in 1508. Since Diana's death, Althorp has become a house and a park known across the world.
When Althorp opens to the public in July and August, the family will be showing the upstairs of the house for the first time since Diana's death. The most dramatic room is the picture gallery, which is 120 feet long. It's the only room left in the house unchanged from the original interior designed 500 years ago.
Also to be open to the public is the King William III room, named after an English king who stayed there 300 years ago as a guest of the family.
In more recent times, says Spencer, it was the favorite guest bedroom of Princess Diana.
"Diana's memory permeates the house," Spencer says.
A painting of her by American artist Nelson Shanks hangs at the top of the main staircase, and there are also family photographs on the top of the piano. But the main dedication to Diana is the exhibition in the half dozen rooms in the stables which encapsulate different aspects of her life.
Spencer says he is especially fond of the home movies on display, one of which shows a very young Diana dancing for the camera.
"It's very moving to see this carefree girl enjoying herself, before the sequence closes with a shot of her looking so very happy, young, and innocent," he says. "It really brings home the tragedy of her premature death 30 years later."
Also on display are a fraction of the condolence books the family has received. Those tributes have come in from all over the world, and there are more than 10,000 of them. At Althorp, there is just a selection of a few of them that the family wants to show the public.
"Most visitors to Althorp want to pay their respects at the island where Diana was laid to rest," says the earl. "She loved this place, she loved the grounds and at the end of the day, she's come home. Diana's element that she really loved was water, and it just seemed the right place for her, in this beautiful island in the middle of this stunning lake. I just think it's the safest and most dignified place she could be."
From the visitors to Althorp, Spencer says he has come to realize that the nature of their mourning is changing as time passes. There are fewer tears and increased interest in learning about history.
"People were accepting that Diana had died," he explains. "They were here more to see the place, to learn more about her. It was less of a pilgrimage and more of an experience for the family to come and learn a bit about her and her family."
H also says, "A lot of the media seem to want to make the public feel stupid for having gone into what they would see as mass hysteria. But my memory of those events is that the emotion was genuine, and the shock was real. And I've always put that down to the fact that if you'd taken a straw poll around the world the day before, as to who the least likely person to die was, it would have been Diana, being young and beautiful and glamorous. It just seemed such a sad, tragic waste when she was killed in a car crash at such a young age. I think people still care very much that she died in this way."
His emotional eulogy for his sister made headlines.
"It was very much a speech written from the heart rather than the head," he explains. "It was honest, and it was exactly how I felt, and I would never take back a word, because it's really what I felt at the time."
But his feelings for the family estate at Althorp have been more complex.
"When I first moved here, and when my father died eight years here, I hadn't been in every room of the house, so it took some getting used to," he explains. "There's 120 rooms here, and it takes some getting used to, I must say."
As he promotes Althorp as a tourist destination, is it tough to do so without seeming to commercialize Diana or to capitalize on her tragedy?
"Well, that is an allegation that's been made. But I have a completely clear conscience on this," he replies. "We have to be open 60 days a year by a decree with the government which preceded Diana's death. And all profits which are independently audited go directly to her memorial fund, so I've invested a lot of money here. I'm not complaining, but I certainly won't see a return on it. I'm doing this out of honoring my sister."
Diana's brother says he never declines any requests from family members who want to visit the island where Diana is interred. As for himself, he rarely visits the island, but frequently finds himself sitting by the lake.
"I go there to reflect on Diana, but also on many other things," he adds. "That's something I have noticed. When visitors come here when they get in the neighborhood of that lake, they tend to sit there and think. It's got an incredible atmosphere around there, and I think people find it a very helpful place to go and contemplate, and I'm no different."
"I was always the younger, the kid brother, and in the next month or so, I'm going to be, for the first time in my life, older than Diana ever was," he says. "And I find that a very unsettling thought, because it's like the natural order of things has gone mad."