Her Royal Highness has organised her customary Christmas carol concert at the iconic London venue, with guests including royals, celebrities and local heroes who have made a difference in their neighbourhoods.
The sizable congregation will listen to a mix of carols, music and readings from actors such as award-winning actors.
The focus of the service, held in the glowing Westminster Abbey on the afternoon, is generosity and expressing affection towards one's neighbour.
Among the guests will be Anna and Jonathan Cordiner, whose child passed away from a brain tumour, and who founded an organisation, a charity called Kayleigh's Wee Stars, to support families who have a terminally ill child.
"The dedication, empathy, and kindness you show, often quietly and unspoken, and without any hope for reward or acknowledgement, make an extraordinary difference to the circumstances of individuals," wrote Catherine in a note prior to the carol service.
The Princess of Wales is continuing her own gradual return to the public eye following her health challenges and treatment, in November making her initial major address in two years, in which she reflected on recognising the "quiet, often invisible work of caring".
Attendees entering the Abbey will be entertained by talented youth performers aided by the charitable trust Future Talent, co-founded by the Duchess of Kent to widen access to instrumental training.
Carys Wood, aged 18, who is a trumpet player and is from Wales, has been supported by the organisation, with funding assisting with instruments, tuition, and transportation expenses - and she said it was "incredible" to be performing at tonight's service.
"Pursuing music is often costly," Carys said. She is now studying at the renowned conservatoire, and added that "each individual ought to be given the possibility" to cultivate their performance abilities.
Eliza, Carys's sister who is 11, who plays the euphonium, will also be at the Abbey. "It's unbelievable that I get to perform here in front of so many notable figures," she said.
Within the audience will be people from various age groups.
That will feature nine-year-old Saphia Turner, recipient of a young hero award, who produces drawings and undertakes charity feats to generate funds for community pantries.
The Abbey has been festooned with specially made wreaths, with TV cook Mary Berry and florist Simon Lycett assisting in their creation.
Among the musical performers will be recording artist Katie Melua, choristers from Westminster Abbey and a Cornish sea shanty group, the group Fisherman's Friends, who said the invitation to sing was "a dream come true."
This annual carol service will be televised and shown on television on December 24th.