The event will be a deeply personal moment for the King, who became sovereign when his mother died peacefully last year at Balmoral on September 8, aged 96, after reigning for 70 years.
The Prince and Princess of Wales will visit the Welsh cathedral of St Davids for a short private service as part of a trip to Pembrokeshire.
The King recorded a message and released a favourite photograph of his mother to mark the first anniversary of her death and his accession to the throne.
Charles, in words written, and audio spoken, at Balmoral Castle and recorded on Thursday, said he recalled with “great affection her long life, devoted service and all she meant to so many of us”.
He said: “I am deeply grateful, too, for the love and support that has been shown to my wife and myself during this year as we do our utmost to be of service to you all.”
The formal colour photograph chosen by Charles was taken by Cecil Beaton and shows her aged 42 in 1968.
She is standing sideways and smiling in her Garter robes and wearing the Grand Duchess Vladimir’s Tiara, made of 15 interlaced diamond circles.
Prime Minister Rishi Sunak also paid tribute to her.
“With the perspective of a year, the scale of her late majesty’s service only seems greater,” he said.
“Her devotion to the nations of the United Kingdom and the Commonwealth only seems deeper. And our gratitude for such an extraordinary life of duty and dedication only continues to grow.”
Mr Sunak said he treasured his memories of meeting the late Queen and was struck by her “wisdom, by her incredible warmth and grace”, and also her “sharp wit”.
“People across the UK, whether they had the good fortune to meet her late majesty or not, will be reflecting today on what she meant to them and the example she set for us all,” he added.
Former Prime Minister Liz Truss has spoken about her final encounter with her, describing how the “upbeat” and frail but “mentally alert” monarch told her they would be “meeting again soon”.
Queen Elizabeth II had welcomed Ms Truss to her home in Aberdeenshire on September 6 2022 to appoint her prime minister.
“She was very, very keen to reassure me that we’d be meeting again soon. It was very important to her,” Ms Truss told GB News.
She added: “She was very determined to do her duty, right to the end.”
Queen Elizabeth II died two days later, with Ms Truss describing the scene as she waited in Downing Street when confirmation came about 4.30pm.
Ms Truss said she felt very sad, but her mind also turned to the practicalities of the days to come.
She recalled the King was “very, very resolute” when she spoke to him to express her condolences on the phone the day his mother died and his reign began.
Charles has been staying at his Birkhall residence and Balmoral Castle in Aberdeenshire over the summer.
He is not expected to have time in his diary to see his son the Duke of Sussex, who flew to London for a WellChild award ceremony on Thursday ahead of the start of the Invictus Games in Germany.
Harry paid tribute to his grandmother in his speech at the event, saying: “I know exactly one year on that she is looking down on all of us tonight, happy we’re together, continuing to spotlight such an incredible community.”
Soldiers and horses who took part in the state funeral procession and proclamation salutes for the new reign will return for Accession Day gun anniversary salutes in the King’s honour on Friday.
Captain Amy Cooper, who was the lead rider in the procession which carried the coffin to lie in state in Westminster Hall, will give the order to fire a 41-gun salute at midday in London’s Hyde Park.
A 62-gun salute will also be fired at the Tower of London by The Honourable Artillery Company – the regiment and the King’s Troop were responsible for gun salutes following the death of Queen Elizabeth II.
Bells will be rung at Westminster Abbey at 1pm in commemoration of the King’s accession.