The annual royal Christmas message is a staple TV viewing for many people, but the King has broken with tradition this year.
Instead of recording the broadcast from a royal residence, Charles, 76, has opted to film his speech at Fitzrovia Chapel, a former chapel of the Middlesex Hospital now closed and demolished.
On Tuesday, the royal family's social media accounts shared a peek from behind-the-scenes of the recording, produced by Sky.
It showed a glimpse of the script for the King's message, as well as the team dressing the Christmas tree with decorations and lights.
Charles was also seen adjusting his tie, as someone calls: "Roll cameras and record. We're rolling, Sir, so whenever you're ready."
Viewers are given a glimpse of the chapel's ornate gold and dark blue Byzantine inspired architecture as the camera pans above His Majesty.
The clip sparked a flurry of comments from royal fans, with one remarking: "Love seeing behind the scenes!"
Another said: "King Charles not only kept up with the tradition but really made it his own. Will definitely be tuning in on Christmas Day. Merry Christmas to you too, Your Majesty."
Many others also revealed they would be watching the King's Christmas message from all over the world, including Australia and South Africa, while others sent festive well-wishes.
The monarch's message will pay tribute to healthcare workers in a year that saw both Charles and his daughter-in-law, the Princess of Wales, treated for cancer.
A major theme of the King's address will also focus on communities who have come together, particularly in difficult times.
It is understood the speech will reflect on international, national and personal challenges and how they can be overcome by communities supporting one another.
Charles recorded his message on 11 December, one of the rare times a building outside the royal estate has been used, with the late Queen Elizabeth II recording her message at Southwark Cathedral in 2006 and at Combermere Barracks in Windsor three years earlier.
The King picked Fitzrovia Chapel, located near Oxford Street, without visiting and was left "absolutely bowled over" by its beauty, said a source.
The speech will be televised on Christmas Day at 3pm GMT on Sky, BBC and ITV, as well as the royal family's YouTube channel.
On the morning of 25 December, Charles will lead the royals on the walk to St Mary Magdalene church on the Sandringham estate, before hosting his family for a traditional turkey lunch.