Elizabeth, Britain's longest-reigning sovereign, died aged 96 on September 8 at Balmoral Castle, her Scottish summer retreat after a 70-year reign.
Charles, who immediately succeeded her as King of the United Kingdom and 14 other countries including Canada, Australia and New Zealand, and his wife Queen Camilla will spend the day quietly at the Scottish royal home.
No ceremonial event is planned to mark his Accession Day but the couple will attend the nearby Crathie Kirk for private memorial prayers.
Buckingham Palace also released a portrait of the late Queen, chosen by Charles, which had previously only been seen in an exhibition.
"In marking the first anniversary of her late majesty’s death and my accession, we recall with great affection her long life, devoted service and all she meant to so many of us," Charles said in a statement.
"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."
Those latter words were an echo of the mantra that Elizabeth repeated throughout her reign.
'Solemn'
While Charles is not due to make a public appearance, his eldest son and heir Prince William and wife Kate will attend a private church service to commemorate the Queen's life.
The Prince and Princess of Wales will visit the 12th Century St Davids Cathedral, a place of pilgrimage for centuries on the Welsh coast which also has a special stall for the sovereign.
There will also be gun salutes fired in a London park and at the Tower of London. Prime Minister Rishi Sunak said the thoughts of the nation would be with Charles and his family on "the solemn anniversary".
"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."
Elizabeth's death marked an end of an era for Britain. During her seven decades on the throne, she had become not just Britain's figurehead but also a huge presence on the world stage.
Charles' first year on the throne has been dominated by his coronation, Britain's biggest ceremonial event for generations, full of pomp and pageantry.
Polls suggest most Britons have a favourable view of his reign so far, although younger generations appear much less enthusiastic.
He is also still dealing with issues within his own family, most notably the rift with his younger son Prince Harry and his wife Meghan, the Duchess of Sussex, and what to do with his own younger brother Prince Andrew, who had to quit royal duties over his friendship with convicted sex offender Jeffrey Epstein.
