Prince William and Prince Harry rededicate Princess Diana's grave on what would have been her 56th birthday

Prince William and Prince Harry are honoring their mother, Princess Diana, on what would have been her 56th birthday.

The Duke of Cambridge and his brother gathered with family on Saturday to pay tribute to their beloved mother, who died in a car crash in Paris, France, on Aug. 31, 1997.

WATCH: Prince William, Prince Harry and Kate Middleton to Rededicate Princess Diana's Grave on Her Birthday

The royals rededicated her grave and held a private memorial service on the island of the Oval Lake at Althorp House in England, the Princess of Wales' family estate in Northamptonshire where she grew up and was laid to rest nearly 20 years ago.

According to BBC News, Prince William's wife, Kate Middleton, was by his side during the emotional tribute, along with their two children, Prince George, 3, and Princess Charlotte, 2, and Diana's brother, Charles Spencer, 9th Earl Spencer. The service was led by Justin Welby, the Archbishop of Canterbury.

Prince Charles -- Diana's ex-husband and Harry and William's father -- was notably absent from the service, as he was performing Royal duties in Ottawa with his wife, Camilla, Duchess of Cornwall, in honor of Canada's 150th birthday, The Telegraph reports.

PHOTOS: Prince William, Prince Harry and Kate Middleton together

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At the time of Diana's death, William was 15 and Harry was 12. The two are hoping to now "stand up for her name" and do their duties "as sons" in protecting her and reminding everyone "of the character and person that she was," according to a new BBC documentary with the working title Diana.

"Part of the reason why Harry and I want to do this is because we feel we owe it to her," William explains in the film. "I think an element of it is feeling like we let her down when we were younger. We couldn't protect her."

"I think it's never going to be easy for the two of us to talk about our mother, but 20 years on seems like a good time to remind people of the difference that she made, not just to the royal family, but also to the world," adds Harry.

Hear more on how they're honoring her legacy in the video above.

WATCH: Prince William Says He Wishes Princess Diana Could Have 'Seen the Children Grow Up'

PHOTOS: Princess Diana with her sons