Catherine, Princess of Wales, is presiding over her customary Yuletide celebration at the iconic London venue, with attendees featuring dignitaries, stars, and everyday helpers who have helped their communities.
The 1,600 guests will hear a blend of carols, music and readings from performers including Kate Winslet and Chiwetel Ejiofor.
The central message of the service, held in the glowing Westminster Abbey on Friday, is generosity and expressing affection towards one's neighbour.
In the audience are a couple named the Cordiners, whose daughter died of a brain cancer, and who established a foundation, a charity called Kayleigh's Wee Stars, to support families who have a child with a terminal illness.
"The time, care and compassion you give, often discreetly and without words, and without any hope for reward or acknowledgement, make an remarkable change to the lives of others," Her Royal Highness stated in a note prior to the festive gathering.
The royal has been undertaking her own measured re-emergence to public life subsequent to her illness and medical care, last month delivering her initial major address in two years, in which she reflected on recognising the "quiet, often invisible work of caring".
Guests arriving at the Abbey will be entertained by talented youth performers supported by the Future Talent organisation, co-founded by the Duchess of Kent to increase availability of musical tuition.
Carys Wood, aged 18, who is a trumpet player and is from southwest Wales, has been assisted by the trust, with awards contributing to musical instruments, lessons and travel costs - and she said it was "amazing" to be playing at tonight's service.
"Music can be very expensive," she stated. She is now a student at the Royal Academy of Music, and added that "each individual ought to be given the possibility" to cultivate their performance abilities.
Her younger sibling Eliza, aged 11, who plays the euphonium, will likewise attend the Abbey. "It's unbelievable that I get to perform here in front of so many distinguished guests," she said.
Attending the service will be individuals spanning multiple eras.
That will feature nine-year-old Saphia Turner, honouree of a youth community award, who creates and sells art and completes fundraising tasks to raise money for local food charities.
The Abbey has been decorated with specially made wreaths, with celebrity baker Mary Berry and floral designer Simon Lycett assisting in their creation.
Taking to the stage will be recording artist Katie Melua, choristers from Westminster Abbey and a nautical singing ensemble, the group Fisherman's Friends, who said the opportunity to perform was "a dream come true."
The festive event titled 'Together at Christmas' will be broadcast on ITV1 on Christmas Eve.
A London-based journalist with a passion for uncovering cultural trends and lifestyle stories across the UK.