It's been 24 years since the People's Princess was tragically killed in a car accident in Paris, and while her sons will be remembering her in private, the brothers have spoken openly about missing their mother several times before.
For Diana's 20th death anniversary in 2017, William and Harry even starred in a couple of TV documentaries, where they spoke extensively about their beloved mum. Let's see their most memorable quotes through the years…
Prince Harry spoke of the lives his mother touched when revealing that he and his brother William have commissioned a 60th birthday statue in her honour. "Our mother touched so many lives. We hope the statue will help all those who visit Kensington Palace to reflect on her life and her legacy," he said.
In an exclusive interview with HELLO! in May 2016, Harry also said he and his brother were determined to create a "permanent memorial" to their mother. "Something needs to be put in stone or in place as a memory," he said. "Lots of people still talk about her. Every single day we still think about her."
WATCH: HELLO! Insider - celebrating Princess Diana
When Harry proposed to Meghan Markle in 2017, talk soon turned to his mother during an interview with the BBC, as he admitted the celebration was bittersweet.
"It is days like… days like today when I really miss having her around and miss being able to share the happy news," he said. "But you know with the ring and with everything else that's going on… I'm sure she's with us yeah, you know, jumping up and down somewhere else."
William was just 15 when his mum died, while Harry was 12.
Speaking in an interview with GQ, Prince William opened up about his sadness that Diana will never know his children. "I would like to have had her advice," he said. "I would love her to have met Catherine and to have seen the children grow up. It makes me sad that she won't, that they will never know her."
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Shortly after announcing his engagement to Kate, Prince William spoke about giving his fiancee Diana's sapphire ring. "It's my mother's engagement ring," he said in their engagement interview. "So I thought it was quite nice. Obviously she's not around to share the fun and excitement of it all so this is my way of keeping her close to it all."
In a BBC1 documentary, Princes William and Harry sat down to discuss their mother. William said: "Part of the reason why Harry and I want to do this is because we feel we owe it to her… I think an element of it is feeling like we let her down when we were younger. We couldn't protect her.
"We feel we at least owe her 20 years on to stand up for her name and remind everybody of the character and person that she was. Do our duties as sons in protecting her."
In the same GQ interview, William spoke about his grief after his mum died when he was just 15. "I am in a better place about it than I have been for a long time, where I can talk about her more openly, talk about her more honestly, and I can remember her better, and publicly talk about her better," he said.
"It has taken me almost 20 years to get to that stage. I still find it difficult now, because at the time it was so raw. And also it is not like most people's grief, because everyone else knows about it, everyone knows the story, everyone knows her."
Speaking in the BBC1 documentary, Harry said: "When she died there was such an outpour of emotion and love which was quite, which was shocking. It was beautiful at the same time, and it was amazing, now looking back at it, it was amazing that our mother had such a huge effect on so many people."
The proud son continued: "When you're that young and something like that happens to you I think it's lodged in here, there, wherever – in your heart, in your head and it stays there for a very, very long time. 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."
The pair spoke about Diana's fun side, with Prince William saying: "She was very informal and really enjoyed the laughter and the fun," before Harry added: "She was one of the naughtiest parents."
In their documentary Diana, Our Mother, Harry elaborated, revealing: "She was one of the naughtiest parents. She would come and watch us play football and, you know, smuggle sweets into our socks.
"And I mean like literally walking back from a football match and having sort of five packets of Starburst. And just the whole shirt was just bulging with sweets and then sort of looking around, open the tuck box, throw it all in, lock it up."
Speaking about if Princess Diana would have been proud of the royals' Head's Together mental health campaign, Prince Harry said: "I hope so. I think she would be proud of the campaign, proud of everybody involved, proud of us. But specifically proud of the UK for having this conversation…
"From her perspective, she would be overwhelmed and hugely encouraged by the fact that the UK has sudden got to this point - and it's only taken a year. And specifically, this week has really energised the whole nation."
Speaking about her passing, William said: "We felt, you know, incredibly loved, Harry and I, and I'm very grateful that that love still - still feels there, even 20 years on. And I think that's a huge credit to her that I - I can still feel that now."
He continued: "I think she'd be proud that Harry and I have managed to come through everything that's happened, having lost her, and that gives me positivity and strength to know that I can, I can face anything the world can throw at me."
During Diana: Our Mother, William joked that she would be a "nightmare" grandmother. He said: "She'd be a nightmare grandmother, absolute nightmare. She'd love the children to bits, but she'd be an absolute nightmare.
"She'd come and go and she'd come in probably at bath time, cause an amazing amount of scene, bubbles everywhere, bathwater all over the place and - and then leave."
Telling stories about their mum during their recent documentary, William said: "She was a massive card writer. She loved the rudest cards you could imagine. And I would be at school and I'd get a card from my mother. Usually she found something, you know, very embarrassing, you know, a very funny card, and then sort of wrote very nice stuff inside. But I dared not open it in case the teachers or anyone else in the class had seen it."
Speaking about a particularly funny memory, William said: "There's a couple of memories I have that are particularly funny. Just outside this room where we are now, she organised, when I came home from school to have Cindy Crawford, Christy Turlington and Naomi Campbell waiting at the top of the stairs.
"I was probably a 12 or 13-year-old boy who had posters of them on my wall and I went bright red and didn't quite know what to say and sort of fumbled, and I think I pretty much fell down the stairs on the way up. I was completely and utterly sort of awestruck.
"But that was a very funny memory that - that's lived with me forever about her loving and embarrassing and sort of, you know being - being the sort of the joker."