Friday's carol concert, the fourth she has hosted, marks Kate's most prominent return to royal engagements since she underwent a course of chemotherapy cancer treatment.
In a letter to the 1600 invited guests, Kate, 42, the wife of heir to the throne Prince William, returned to themes of love and the need for empathy about which she has spoken in previous very personal statements and video updates on her health.
View this post on Instagram A post shared by The Prince and Princess of Wales (@princeandprincessofwales)
Christmas was not only a time for celebration, but also for reflection and relief from the pressures of daily life, she said.
The Christmas story reflected "our own vulnerabilities", and how much people needed each other despite their differences, she said.
"Above all else it encourages us to turn to love, not fear," she wrote.
"It is this love which is the greatest gift we can receive. Not just at Christmas, but every day of our lives," she said.
William, who earlier this month said the year had been "brutal" for the royal family with Kate's treatment coming on top of his father King Charles' own cancer diagnosis, will give a reading at the service.
Prince William will do a reading at the service. (AP PHOTO)
Six-time Olympic track cycling champion Chris Hoy, who revealed he had terminal cancer in October, will light a candle.
The 'Together At Christmas' service will be broadcast on Britain's ITV on Christmas Eve.
