Commonwealth Day is one of the rare occasions all senior royals join together for a grand service at Westminster Abbey.
And tomorrow's ceremony will be a poignant one as it will mark the first King Charles has attended as monarch.
He will be joined by wife Queen Camilla as well as eldest son Prince William and the Princess of Wales - but not Prince Harry and Meghan Markle.
In fact, the event in 2020 marked their last royal engagement before quitting as senior royals to carve out new lives for themselves in North America.
They attended the service alongside other members of the Firm - and many remarked on the tension between the family members.
And according to a book, Harry appeared to be "fighting back the tears" following what they perceived to be a "snub".
Before the ceremony, 2,000 orders of service had been distributed detailing that William and Kate would enter the Abbey with the main royal party.
However, there was no mention of Harry and Meghan being in the procession, with the Daily Mail reporting the couple was "quite sensitive and emotional about it".
This reportedly led to William and Kate not walking in the procession in an attempt to resolve the issue - despite the orders of service stating otherwise.
And in his book Battle of Brothers, royal author and historian Robert Lacey says even though Meghan had a "megawatt" smile, Harry was more gloomy.
He writes: "Observers also noted that Harry's face was 'quite tense and unsmiling' - and that when William sat down close to him, he barely greeted his brother.
"Throughout the service, Meghan megawatted away with her best TV smile but, as the ceremony progressed, Harry appeared to grow gloomier.
"According to one observer, 'his accelerated blinking even suggested he might have been fighting back tears.'"
Harry later told how he felt at the service in his and Meghan's controversial Netflix series released at the end of last year.
The service was the first time he and Meghan had seen many family members since an extended Christmas stay in Canada - and the news broke about them quitting the Firm.
Harry explained: "The first time that we saw the other members of the family was in Westminster Abbey.
"We were nervous seeing the family because all the TV cameras and everybody watching at home and everybody watching in the audience. It's like living through a soap opera where everybody else views you as entertainment.
"I felt really distant from the rest of my family, which was interesting because so much of how they operate is about what it looks like, rather than what it feels like. And it looked cold. But it also felt cold."
The Commonwealth Day theme for 2023 is Forging a Sustainable and Peaceful Common Future and comes at a time when Charles has experienced great turbulence in his relationship with his youngest son, the Duke of Sussex.
Afterwards, Charles, Camilla and the attending members of the royal family will entertain the Commonwealth secretary-general, high commissioners, foreign affairs ministers and other members of the Commonwealth community at a Buckingham Palace reception.
