It’s been nearly 20 years since Princess Diana‘s tragic August 1997 death — and now, a new TV special is providing the public with never-before-heard details of Diana’s life and passing.

In Diana, Our Mother: Her Life and Legacy — which airs tonight on HBO at 10 p.m. EST — the People’s Princess’ two adult sons, Prince William and Prince Harry, candidly opened up about their royal upbringing and personal memories of Diana. Scroll down to see everything William and Harry newly revealed in the emotional documentary.

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William and Harry hadn’t seen their mom for a month before her death

The young princes and Diana were unfortunately separated by pre-scheduled trips — William and Harry were on vacation with their father, Prince Charles, and Diana was doing humanitarian work — for the month leading up to her untimely death. In Diana, Our Mother: Her Life and Legacy, Harry revealed he and his brother were supposed to reunite with their mother on Aug. 31, the day Diana unexpectedly passed away.

“There was the point where our parents split and… we never saw our mother enough or we never saw our father enough,” Harry said. “There was a lot of traveling and lot of fights on the back seat with my brother – which I would win… It was an interesting way of growing up.”

The brothers still regret their last phone call with Diana today

At the time of Diana’s passing, William was 15 years old and Harry was just age 12. So, when their mother telephoned the brothers to catch up on Aug. 31 — the day she died — William and Harry cut the conversation short because they wanted to get back to playing with their cousins. “I can’t necessarily remember what I said, but all I do remember is regretting for the rest of my life how short the phone call was,” Harry said in the documentary.

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“If I’d known that that was the last time I was going to speak to my mother, the things I would have said to her. Looking back at it now — it’s incredibly hard. I have to deal with that for the rest of my life: not knowing that it was the last time I’d speak to my mom, how differently that conversation would have panned out if I’d had even the slightest inkling that her life was going to be taken that night,” he continued.

“[We were] running around, minding our own business, playing with our cousins and having a very good time. [We] were in a rush to say, ‘goodbye, see you later, can I go off?'” William said. “If I’d known what was going to happen I wouldn’t have been quite so blasé about it. That phone call sticks in my mind quite heavily.”

William felt Diana “was there” at his 2011 royal wedding

“I did really feel that she was there… there were times I looked to someone or something for strength — and I very much felt she was there for me,” William said of his April 2011 nuptials to Kate Middleton.

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Diana would have been a “nightmare” grandmother today

Sadly, Diana never got to meet her two grandchildren, Prince George and Princess Charlotte — but, according to William, his mother would be a “nightmare” grandma today if she were alive. “We’ve got more photos up around the house now of her and we talk about her a bit and stuff. It’s hard, because obviously [Kate] didn’t know her, so she cannot really provide that, that level of detail,” William began.

“So, I do regularly when putting George or Charlotte to bed, talk about her and just try and remind them that there are two grandmothers — there were two grandmothers — in their lives. So, it’s important that they know who she was and that she existed,” he continued. “She’d be a nightmare grandmother — absolute nightmare. She’d love the children to bits, but she’d be an absolute nightmare.”

“She’d come and go and she’d come in probably at bath time, cause an amazing amount of scene, bubbles everywhere, bathwater all over the place, and then leave,” William joked.

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