Princess Diana

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HBO's documentary Diana, Our Mother: Her Life and Legacy promises to be emotional, considering Prince William and Prince Harry will open up about memories of their mother, Princess Diana. But there's one memory in particular the men wish they could change: the last time they spoke with her.

At the time of her death in 1997, the princes hadn't seen their mother in a month, since they were spending the summer in Scotland with their father, Prince Charles, and grandmother, Queen Elizabeth II, while Diana was vacationing in Paris. The boys, who were just 15 and 12 years old at the time, spoke to Princess Diana on the day of her death; they were playing outside when she called. And as reported by the Washington Post, 20 years later they still have regrets about that phone call, according to a new trailer for the show.

“Harry and I were in a desperate rush to say goodbye, you know, ‘See you later’.… If I’d known now, obviously, what was going to happen, I wouldn’t have been so blasé about it and everything else,” Prince William says in the documentary. “But that phone call sticks in my mind, quite heavily.”

Harry added, “Looking back on it now, it’s incredibly hard. I’ll have to sort of deal with that for the rest of my life. Not knowing that was the last time I was going to speak to my mum. How differently that conversation would have panned out if I’d had even the slightest inkling her life was going to be taken that night.”

In recent months, both William and Harry have been vocal about mental health advocacy and how they dealt with their own grief following Princess Diana's death. Harry has said that he's "probably been very close to a complete breakdown on numerous occasions" over the course of the last 20 years.

Though the documentary, which will air on July 24 on HBO at 10 P.M., treads into some deep and heartbreaking material, it also promises some happier memories of the People's Princess, including funny never-before-told anecdotes. Princess Diana was known for her humor, and Harry says one of her "mottos" was "You can be as naughty as you want, just don't get caught."

The princes will also discuss their family life, as well as Diana's public life and her work to fight homelessness, HIV, and AIDS. A few of Diana's friends and family members, some of whom have never before spoken publicly about her, will also be featured. But there's one thing that comes across no matter who is telling the story: Diana loved her sons more than anything. William says in a trailer for the show, "We felt very loved by her, Harry and I. And I'm grateful that that love still feels like it's there."

Related: Prince William and Kate Middleton Tried to Get Game of Thrones Spoilers Out of One of the Show's Actors

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