The King is to attend church near Balmoral for private prayer and reflection in remembrance of his mother Queen Elizabeth II on the second anniversary of her death.
Charles will head to Crathie Kirk, where the late Queen worshipped, for Sunday morning service, two years on from his accession to the throne.
The King, 75, has been staying on his Aberdeenshire estate in the Highlands, where his mother died peacefully at the age of 96 on September 8 2022.
The King’s second year as sovereign was marked by the difficult double health blow of his cancer diagnosis and that of his daughter-in-law the Princess of Wales.
But with the monarch and Queen consort travelling in the autumn to Australia and Samoa on their first major long-haul trip since news of his condition broke, a royal source said Charles’s health was “heading in a positive trajectory”.
On Accession Day, the Archbishop of Canterbury, who delivered the sermon at the late Queen’s state funeral, has given thanks for Elizabeth II’s “life of faith and devotion” and prayers for the King’s “steadfast service”.
“As we pray for the royal family today, we give thanks for Queen Elizabeth’s life of faith and devotion – and continue to pray for the King in his steadfast service to our nation.”
A royal source said: “There will be personal thoughts, prayers and reflections.
“I’m sure it will be a very poignant day for all. How could it not be, not least given the year of challenge that the whole family has had?”
“Nobody saw those things coming and they had a profound impact,” Mr Little said.
“It has been a rollercoaster year for the King, not just for him, but for his family, and it will have been of great concern to the Queen.
“Nonetheless he has been remarkably active for someone with his condition and given his age.
“He hasn’t really let the treatment hold him back, but I’m sure he hasn’t done anything near as much as he would have liked to have done.”
The King’s second year as monarch has been less hectic than his first, with cancer treatment understandably having an impact on his schedule, though he has still undertaken dozens of visits, meetings and events.
On Saturday, Charles, dressed in a tartan kilt, and Camilla enjoyed a day out at the annual Braemar Gathering, watching the Highland Games.
Scotland’s First Minister John Swinney and his wife Elizabeth Quigley also arrived to stay overnight with the King and Queen the same day, with the King holding an audience with Mr Swinney on Saturday evening.
Prayers will be said for the King and the royal family hundreds of miles away at services in London’s Westminster Abbey – the scene of the late Queen’s final farewell at her funeral 11 days after her death.
Gun salutes are usually fired on Accession Day, but because this year it falls on a Sunday when salutes traditionally do not take place, the military tribute by The King’s Troop Royal Horse Artillery at Hyde Park and The Honourable Artillery Company at the Tower of London has been moved to Monday.
Both regiments taking part were responsible for firing the Death Gun salutes upon the death of Elizabeth II, with almost all of the soldiers and horses taking part playing a role in the Queen’s funeral and the proclamation of the King.
Members of the royal family regularly worship at Crathie Kirk while they are staying at nearby Balmoral Castle, and the late Queen, a devout Christian, was a regular attendee.
The tiny grey granite place of worship, a short drive from the castle, stands on a hill overlooking 14th-century church ruins and the River Dee.
The Princess Royal married then-commander Tim Laurence there in the winter of 1992.
