William and Harry also made an impromptu visit to speak with well-wishers and view the tributes and cards left at the gates of Kensington Palace, where Princess Diana lived from 1981 until her death.
"The Duke and Prince Harry are grateful for the many flowers, letters, and messages they have received about their mother," Kensington Palace said. "They wanted to say thank you to those who made the journey to Kensington Palace."
Also in attendance were representatives from the Royal Marsden Hospital, where William now serves as royal patron, and the National Aids Trust, which Prince Harry has supported with his own AIDS charity, Sentebale, that he set up in his mother's name.
A team of six gardeners and a number of volunteers spent 18 weeks planting the floral tribute of white roses, white Diana tulips, white hyacinth, forget-me-nots and other favorites of Diana. The "White Garden," as it is known, opened in April.
William and Harry were just 15 and 12, respectively, when their mother died on Aug. 31, 1997, at age 36, due to injuries sustained in a car crash in Paris.
William, Harry, and Kate are now trying to keep Diana's memory alive for her grandchildren, Prince George, 4, and Princess Charlotte, 2.
On July 3 -- what would have been Diana's 56th birthday -- William and Harry held a service of re-dedication at Diana's grave on the island in Round Lake at Althorp, the Spencer family home. George and Charlotte also attended the service.
Describing how he keeps his mother's memory alive, William said earlier this year that he speaks about "Granny Diana" to his children, adding: "So 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, Catherine didn't know her, so she cannot really provide that level of detail," he said in a documentary for ITV that aired in July. "So I do regularly, 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 in their lives, and so it's important that they know who she was and that she existed."
In addition to the garden, Kensington Palace has hosted a number of events to commemorate Diana's life this year.
An exhibition chronicling Diana's evolving style during her life opened at Kensington Palace in February. The exhibition, titled "Diana: Her Fashion Story," offers a unique look at Diana's style and features some of her most stunning outfits.
William and Harry have commissioned a statue to honor Diana on the grounds of the palace, where Diana lived until her death.
"Now all I want to do is try and fill the holes that my mother has left, and that's what it's about for us, is trying to make a difference and in making a difference, making her proud," Harry said in a BBC documentary that aired this month. "She was the Princess of Wales, and she stood for so many things, but deep down inside for us, she was a mother. And we will miss our mother, and I wonder every single day what it would be like having her around."