The day she died, they were due to see her after nearly a month.
The world remembers how shocking the death of Princess Diana was, especially as she was leaving behind two young princes. But the memory of that fateful day is made even more shocking by the fact that both Prince Harry and Prince William hadn't seen their mother for almost a month before she passed away.
Prince Harry and Prince William play polo together
Prince Harry has talked candidly about his mother's death on many occasions recently, confessing that he had to have counselling in recent years after suppressing his grief for so long. But part of that grief no doubt was the thought that they were due to reunite with Diana on the very day that she was involved in a fatal car accident.
The royals expressed their sadness and talked about their memories in a series of special interviews ahead of the 20th anniversary of the tragic events. They had been separated from their mother for several weeks following their return from the St. Tropez villa that belonged to business magnate Mohamed Al Fayed and his son - and Diana's new lover at the time - Dodi Fayed.
After that, she embarked on a charity venture before embarking on a cruise with her new boyfriend, while Harry and William stayed with their father Prince Charles and grandmother Queen Elizabeth II in Balmoral. In a new documentary entitled 'Diana, Our Mother: Her Life and Legacy', they revealed that the last time they spoke to their mother was over the phone on the day that she died.
It had been a short conversation because they were more interested in playing out at the time. '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', said Harry.
He also opened up about how tough Diana and Charles' separation was. 'There was the point where our parents split and... we never saw our mother enough or we never saw our father enough', he explained. '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.'