Prince William and Prince Harry didn’t see their mother Princess Diana for about a month before she died in August 1997.

The princes and their mother were separated because of a vacation they were taking with their father Prince Charles and Diana’s humanitarian trips abroad. They had been on holiday with Diana as guests of the Al Fayed family earlier that summer.

They were due to meet up with their mother on August 31 – the day that she died, aged just 36, in a car crash alongside boyfriend Dodi Fayed in a Paris underpass.

In a scene from their new documentary for ITV and HBO, Diana, Our Mother: Her Life and Legacy, Harry reveals that they hadn’t seen their mother for several weeks while speaking with two landmine victims, Zarko Peric and Malic Bradaric, who Diana met when she went on her last mission to Bosnia to raise awareness about the brutality of landmines.

“You guys were the almost the last people to see my mother,” Harry tells them in the documentary, which airs in the U.K. and the U.S. on Monday. “Well, you saw my mother more recently than I did, I guess.”

William and Harry had vacationed with Diana alongside the Fayed family earlier in the summer before they headed to Balmoral, Scotland, to spend time with their father and other members of the royal family, including Queen Elizabeth II.

Like many other children of separated and divorced parents, they also spent time traveling between the two homes in the years before Diana’s death.

“There was the point where our parents split and . . . we never saw our mother enough or we never saw our father enough,” Harry says in the documentary. “There was a lot of traveling and lot of fights on the back seat with my brother – which I would win.

“There was all that to contend with. And – I don’t pretend we’re the only people to have to deal with that, but it was an interesting way of growing up.”

The documentary features candid conversations with the princes and Diana’s friends, family and admirers. The discussion Harry and William have about Diana in the new special are deeply personal, including them sharing their regret over the last time they spoke to their mother before her death.

“If I’d known that that was the last time I was going to speak to my mother the things that I would – the things I would have said to her,” Harry said about their final phone call when he was just 12.

Diana, Our Mother: Her Life and Legacy will air Monday at 10 p.m. on HBO in the U.S. and 9 p.m. in the U.K. on ITV.

The show is one of several commemorations of the Princess in print and on film. PEOPLE and ABC have partnered in a two-night television event The Story of Diana, airing on Aug. 9 and 10 at 9 p.m. ET.