Catherine, Princess of Wales, is presiding over her annual Yuletide celebration at the iconic London venue, with guests including royals, celebrities and local heroes who have helped their communities.
The large audience will enjoy a mix of traditional hymns, orchestral pieces, and literary excerpts from stars like Kate Winslet and Chiwetel Ejiofor.
The focus of the service, held in the glowing Westminster Abbey on the afternoon, is compassion and demonstrating care for others.
Among the guests will be Jonathan and Anna Cordiner, whose child passed away from a tumour, and who founded an organisation, a charity called Kayleigh's Wee Stars, to help households where there is a terminally ill child.
"The time, care and compassion you give, often discreetly and without words, and without any thought of thanks or praise, make an profound impact to the well-being of people," wrote Catherine in a message before the festive gathering.
The royal has been undertaking her own steady comeback to public life after her cancer diagnosis and chemotherapy, in November making her initial major address in two years, in which she discussed recognising the "selfless, commonly overlooked efforts of carers".
Attendees entering the Abbey can listen to young musicians supported by the Future Talent organisation, set up with the help of the Duchess of Kent to increase availability of instrumental training.
Trumpeter Carys Wood, 18, who performs on the trumpet and is from Pembrokeshire, has been helped by the organisation, with funding assisting with musical instruments, lessons and travel costs - and she said it was "incredible" to be taking part in the concert.
"Pursuing music is often costly," Carys said. She is now enrolled at the prestigious music academy, and added that "everyone should have the opportunity" to develop their musical skills.
Carys's 11-year-old sister, Eliza, who is a euphonium player, will also be at the Abbey. "I never imagined I would perform in front of so many notable figures," she said.
Within the audience will be individuals spanning multiple eras.
That will feature young Saphia Turner, nine, recipient of a youth community award, who creates and sells art and completes fundraising tasks to generate funds for local food charities.
The Abbey has been festooned with custom-created holiday decorations, with TV cook Mary Berry and florist Simon Lycett helping to make them.
Among the musical performers will be Katie Melua, choristers from Westminster Abbey and a Cornish sea shanty group, the group Fisherman's Friends, who said the chance to take part was "a dream come true."
The Together at Christmas carol concert will be aired by ITV1 on Christmas Eve.
A seasoned gambling analyst with over a decade of experience in casino strategy and odds optimization.