The beautiful singer was as chic as always in a vintage nude top with a black lace overlay and oversized sleeves, teamed with a high-waisted baby pink PVC midi skirt from H&M.
Katie will perform tomorrow night in front of 4,000 people including Her Majesty The Queen at the iconic London concert hall. It will be an evening close to her heart as she travelled to Afghanistan earlier this year to entertain the troops.
"Performing at the Festival of Remembrance means a great deal to me," she said. "I got involved in supporting our troops at the Queen Elizabeth Hospital in Birmingham last year. The Poppy Appeal is an incredible campaign to be involved in. Working with the Armed Forces has been a life changing experience."
Katie spent three days visiting soldiers in Helmand Province in February and said at the time: "It felt like such a special gig - all the soldiers were there and it felt really incredible." She sang for injured soldiers at the Queen Elizabeth Hospital the previous December.
Aside from Katie, the audience will hear performances by James Blunt, who is a former Army Officer, the Band of the Royal Marines and the Central Band of the Royal Air Force.
The Poppy Girls will also be there. Each member of the young girl group comes from a Forces family. They will perform their single 'The Call (No Need To Say Goodbye)', which is raising money for the Poppy Appeal.
If you don't have a ticket you can watch the entire Festival live on BBC1 from 9.15pm and Radio 2 from 8pm.
Musical performances will be interspersed with real life stories and accounts from members of the Forces.
Chris Simpkins, Director General of The Royal British Legion said: "The festival will provide a moving and engaging show here in London and for viewers across the country, but more importantly it helps us to reach out to wider audiences and offer the public an opportunity to pay their respect, and remember those who, in past and present conflicts, have made sacrifices in the course of duty.
"We're honoured to welcome our patron Her Majesty The Queen, as well as so many Forces families to the event."
The Festival of Remembrance honours the services and sacrifices of British Armed Forces. It will also commemorate the 70th anniversaries of the Battle of the Atlantic and the Dambuster Raids.
The following day, Sunday, the annual Remembrance Day service will be held at the Cenotaph with a two-minute silence at 11am to mark 11 November's Armistice Day.
Yesterday, members of the Royal Family were out and about in London lending their support to the London Poppy Day. The Duke and Duchess of Cambridge surprised commuters at High Street Kensington tube station when they stopped by to chat with poppy vendors, while Prince Harry and the Duke of Edinburgh paid their respects at Westminster Abbey's Field of Remembrance.