Princes William and Harry were young when Princess Diana died in a car crash on August 31, 1997, in Paris. A new documentary "Diana, Our Mother: Her Life and Legacy" to be aired on HBO in the U.S. and ITV in the U.K. Monday night marks the 20th anniversary of Diana’s tragic death and reveals their grief, especially over the last call they received from their mother. It also reveals the fun and naughty side of their much-missed mother.

Princes speak out about Princess Diana’s untimely death

The whole world was shocked at the untimely death of Princess Diana while traveling in Paris, but it was worse for her two sons, William and Harry.

In the documentary, they speak up about their grief over the loss of their mother, particularly her last phone call from Paris. Prince Harry said the call ended up being so brief that he will regret it for the rest of his life.

Both William and Harry will forever regret cutting short Diana’s last call

William was 15 years of age and Harry only 12 when their mother was killed in the car crash. William recalled how he and Harry were having a great time on the day Diana called, playing with their cousins at Balmoral, Queen Elizabeth’s Scottish residence. In fact, they were having so much fun, both he and Harry were in a hurry to get off the phone and return to their playing.

William said it was a case of “see you later,” as they ended the brief call. He said if he had known then what he knows now, he wouldn’t have been so blasé over the call that day, saying that phone call will forever stick in his mind.

In the documentary, William is asked whether he recalls what their mother had said to him at the time, which he said he did, but would not tell the interviewer exactly what the conversation included.

Harry then spoke about his brief chat with his mother. As reported by Metro, he can’t really recall exactly what he said, but what he does remember is that he will regret how short the call was for the rest of his life. Harry did add that he only cried twice for Diana. One time was at his mother’s funeral and the other at an occasion which he did not disclose.

Princess Diana’s fun and naughty side

As noted by the Radio Times, there are, reportedly, happier moments in the documentary, where William and Harry describe Princess Diana as being the “best mother ever.” William said Diana brought a “breath of fresh air” into everything she ever did. Both princes described the mischievous side of Diana, saying she was sometimes saucy and occasionally embarrassing, but always loving to her family.

William adds that there are few days that go by when he doesn’t think about his mother, but that the 20th anniversary of her death is a good time to remember the good things about Diana and offer people a different side to her they have never seen before.

The documentary also outlines Diana’s personal journey and her various campaigns to help AIDS victims and the homeless and wishing to ban landmines in the world.

The trailer for "Diana, Our Mother: Her Life and Legacy" is included below.