Charles III turns 75, showing no signs of slowing down just over a year since becoming king following the death of his mother, Queen Elizabeth II.
The British monarch will spend the day in public engagements, followed by a private dinner at his residence in London.
The lifelong environmentalist will use the day to highlight causes close to his heart, including a visit to a surplus food distribution center with his wife Queen Camilla.
During the visit, he officially launched the Coronation Food Project, an initiative aimed at tackling food shortages by redistributing food that would otherwise end up in landfill.
Charles will also host a reception at Buckingham Palace for 400 nurses and midwives as part of this year's celebrations of the 75th anniversary of the government's National Health Service (NHS).
The NHS Choir will greet him with a surprise birthday song and cannon salutes will be fired in London and across the UK, Buckingham Palace said.
Camilla, 76, once revealed that the notoriously workaholic royal is particularly picky when it comes to gifts.
"I'll tell you, he's the hardest person in the world to buy a present for ... he likes to make a list of things he wants, so you have to get it right," she said.
He likes "cake and a bit of singing," she said, but added that it's often hard to get him to rest.
The evening celebration will be attended by close family and friends, although his estranged younger son Harry will be missing.
A spokesman for Harry and his American wife Meghan denied reports that they had turned down the king's invitation"
Harry, 39, and Meghan, 42, left their royal duties in 2020 and moved to California.
They have since unleashed a barrage of criticism of the royal family, leading to a strained relationship with his father and a rift between Harry and his older brother, Crown Prince William.
Prince Charles Philip Arthur George was born on 14 November 1948 at Buckingham Palace, the first child of the future Queen Elizabeth II and Prince Philip, Duke of Edinburgh.
When he turned 70 in 2018, Charles joked that it was "worrying" and that he had acquired "all the scars that come with getting older.
Charles celebrates his 75th birthday in the same year he was crowned, and just a week after opening the UK Parliament for the first time as sovereign.
Like his mother, who died aged 96 in September 2022, Charles maintains a busy schedule of royal duties despite his advancing years.
But Ed Owens, a royal historian and author, told AFP that Charles had taken a more active role on the international stage than the late Elizabeth.
Charles has accepted the role of "a kind of international leading diplomat for Britain" and the Commonwealth.
He also showed that he was happy to speak openly about difficult issues related to colonialism and the British Empire.
On a visit to Kenya earlier this month, Charles admitted there was "no excuse" for the colonial-era abuses committed in the East African country.
"He confronts some of these more problematic stories in a way that Elizabeth II never would," Owens added. /BGNES
