The princes will tour Diana's memorial garden at Kensington Palace together with representatives from the charities she supported, including those helping AIDS sufferers and children in need.
"The engagement will allow the Princes to pay tribute to the life and work of their mother the day before the 20th anniversary of her death," said a spokeswoman for Kensington Palace where the princes now live.
"Together, they will reflect on the significant achievements of the Princess, and the legacy of her work which continues to resonate with so many today," she said.
Diana's untimely death two decades ago on Thursday shocked the world.
"She was this ray of light in a fairly grey world," 35-year-old William, her eldest son and second in line to the throne, said in a new documentary for the anniversary.
The life of Diana -- a shy, teenage aristocrat who suddenly became the world's most famous woman -- and her tragic death at 36 still captivates millions across the globe.
Two decades on, her sons William and Prince Harry only now feel able to talk publicly about her death, a seismic event which continues to resonate in the monarchy and British society.
Diana died in a car crash in Paris in the early hours of August 31, 1997, along with Dodi Fayed, her wealthy Egyptian film producer boyfriend of two months, and a drink-impaired, speeding driver Henri Paul, who was trying to evade paparazzi.
No public events are planned for Thursday.
No more stiff upper lip
Candles, bouquets of flowers and pictures from well-wishers are already building up at the palace gates, while hardcore Diana fans have turned up with a cake bearing her picture.
But the scene is nothing like the sea of flowers laid in the week between her death and her funeral: an outpouring of national grief that commentators are still grappling with.
William and Harry have spoken of struggling to comprehend the "alien" wave of public mourning among people who didn't know their mother, at a time when they, aged 15 and 12, could not process their loss.
Britain, the nation of the stiff upper lip, was now wailing and hurling flowers at a hearse.
A public that had lapped up every twist and turn in the life of their fairytale princess was crying for their lost icon, killed in the chase for the next day's pictures.
Diana married Prince Charles, the heir to the throne, in 1981, but their marriage collapsed under the strains of public duty and their incompatibility.
Fragility and steel
The monarchy's shining star, now a fashion icon, humanitarian and self-styled "queen of hearts", found herself cast out of the royal family in the 1996 divorce she did not want but had made inevitable with an unprecedented and explosive television interview.
With all her mistakes and frailties played out in public, Britons felt wrapped up in Diana's life.
A testament to her enduring impact, British newspapers have been filled this month with special pull-outs and magazine articles covering all facets of Diana's life.
The complexity in Diana's character still fascinates.
Her strange combination of fragility and steel, her need for loving attention and an ability to make those she met feel loved.
She also had a caring nature mixed with a vengeful streak and the air of being an ordinary girl who was actually a blue-blood aristocrat.
Granny Diana
William has said he tells his children, Prince George, four, and Princess Charlotte, two, about the Granny Diana they will never meet.
Perhaps her greatest legacy is her two sons.
"When you have something so traumatic as the death of your mother when you're 15, it will either make or break you," William said in the "Diana, 7 Days" BBC documentary.
"And I wouldn't let it break me. I wanted it to make me. I wanted her to be proud of the person I would become."
Harry added: "We will miss our mother and I wonder every single day what it would be like having her around."
