Her Royal Highness has organised her annual Yuletide celebration at the iconic London venue, with attendees featuring members of the royal family, famous faces, and community champions who have made a difference in their neighbourhoods.
The 1,600 guests will listen to a blend of carols, music and readings from actors such as Kate Winslet and Chiwetel Ejiofor.
The central message of the concert, held in the atmospheric historic church on Friday afternoon, is compassion and demonstrating care for others.
In the audience are a couple named the Cordiners, whose daughter died of a brain cancer, and who established a foundation, a support fund, to help households where there is a terminally ill child.
"The time, care and compassion you give, often without fanfare or recognition, and without any hope for reward or acknowledgement, make an profound impact to the circumstances of individuals," Her Royal Highness stated in a note prior to the carol service.
The princess has been making her own measured re-emergence to the public eye after her cancer diagnosis and chemotherapy, last month delivering her first public speech for two years, in which she reflected on recognising the "quiet, often invisible work of caring".
Those coming to the Abbey can listen to talented youth performers aided by the Future Talent charity, co-founded by the Duchess of Kent to widen access to music lessons.
Trumpeter Carys Wood, 18, who is a trumpet player and is from Pembrokeshire, has been assisted by the trust, with grants helping towards instruments, tuition, and transportation expenses - and she said it was "awesome" to be playing at tonight's service.
"Music can be very expensive," the musician remarked. She is now a student at the renowned conservatoire, and added that "everyone should have the opportunity" to hone their artistic talents.
Carys's 11-year-old sister, Eliza, who performs on the brass instrument, will be present at the Abbey. "I never thought I'd be able to play there in front of so many notable figures," she said.
Among the guests will be individuals spanning multiple eras.
That will feature nine-year-old Saphia Turner, honouree of a youth community award, who sells artwork and carries out sponsored challenges to raise money for food banks.
The Abbey has been adorned with handcrafted festive wreaths, with TV cook Mary Berry and florist Simon Lycett assisting in their creation.
The musical lineup includes recording artist Katie Melua, choristers from Westminster Abbey and a Cornish sea shanty group, the Fisherman's Friends, who said the chance to take part was "the perfect culmination."
The Together at Christmas carol concert will be broadcast on ITV1 on Christmas Eve.
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