Princes William & Harry reveal Princess Diana's private humor and their last convo with her in HBO doc

In a new HBO documentary, Prince William and Prince Harry come together to give their first-ever joint interview about their late mother, Princess Diana, who died 20 years ago on Aug. 31, 1997.

Diana, Our Mother: Her Life and Legacy, premiering Monday, July 24 at 10 p.m. ET/PT, offers a touching look at the late royal's whirlwind life, from her youthful days wearing men's jerseys and reading trashy romantic novels to becoming the "people's princess" to her lasting impact years beyond her tragic death.

While the 65-minute film features interviews with Sir Elton John, Rihanna, Diana's family and childhood friends as well as her former employees, it's the conversations with her two sons that are most revealing.

"She was our mum. She still is our mum. And of course, as a son I would say this, she was the best mum in the world...She smothered us with love, that's for sure," Harry says early on.

While looking at never-before-seen photos taken from an album Diana assembled while she was still alive, the two share untold stories and memories.

"Behind closed doors, she was a loving mother and an incredibly funny person," Harry says at one point. In fact, both William and his brother recall their mother's sense of humor only seen by them.

MORE: Prince William and Prince Harry Open Up About Princess Diana's Parenting Style

For Harry, the one thing that sticks with him the most is her laugh. "All I can hear is her laugh in my head," he says, adding that one of her mottos to him was, "You can be as naughty as you want, but just don't get caught."

"She was the naughtiest parent," he adds.

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Meanwhile, William recalls her sending ridiculous cards to his school, causing him to get nervous to open them in public. But one moment that left him floored was when he was 12 or 13 years old.

"She organized -- when I came home from school -- to have Cindy Crawford, Christy Turlington and Naomi Campbell waiting at the top of the stairs," he recalls. "I went bright red and didn't know what to say... I was completely and utterly awestruck."

While Diana lived a very public life, speaking out on issues and dedicating her time to humanitarian work, Harry says that she turned to her two sons for escape. "You need respite, somewhere to go to dump it," he says. "I think she lived a lot of her life, especially in private, through us."

"Our mother was a total kid through and through," he continues, revealing that she would sneak them out to movies and take them on long drives listening to Enya ("That's a blast from the past.") all so they could live normal lives.

"She realized there was life outside of palace walls and she wanted us to see that from a very young age," William says of his mother introducing him to her humanitarian work and opening their eyes to the world.

Of course, Diana's fairy-tale life nearly crumbled following her separation from husband Prince Charles. The divorce made her an even bigger target of the paparazzi. "I sadly remember the most she ever cried was to do with press intrusion," William says of family vacations ruined by cameras. "People would be utterly appalled if they knew what was going on."

"One lesson I've learned is you never let them in too far because it's very difficult to get them back out again," he says of keeping a distance between the cameras and his personal life.

When it comes to her final days, both William and Harry regret not having taken full advantage of it before she died. "If I'd known now what was going to happen, I wouldn't have been so blasé about it," William says, referring to the last time he spoke to her. "Our phone call sticks in my mind quite heavily."

"I can't necessarily remember what I said. All I do remember is regretting for the rest of my life how short the phone call was," Harry recalls. "Looking back on it now is incredibly hard."

As the documentary shifts focus to the princes' lives after Diana's death, William says "it spins you out ... but slowly you try to rebuild your life."

"People deal with grief in different ways and my way of dealing with it was shutting it out and locking it up," Harry says. "I went through a period of sorting myself out."

Now, both continue to keep Diana's legacy alive by continuing her humanitarian work and taking active roles in her biggest causes. For William that also means "constantly talking about Granny Diana" to his two kids, George and Charlotte, with wife Duchess Kate Middleton.

"I want to make enough time with George and Charlotte as I can," he says of learning from what his mother did for him and his brother.

"It has been hard and it will continue to be hard. There's not a day William and I don't wish she was around," Harry concludes.