In Sarah Ferguson's autobiography, she outlined the many changes of outfit that the Royal Family go through during Christmas Day at Sandringham, and first up during her time as an HRH was a long tweed skirt, blouse, and cardigan for breakfast.
Thirty years later, Kate’s modern take would likely be something like the monochrome bouclé tweed Dolce & Gabbana mini skirt that she first wore to guest-edit The Huffington Post, and a second time for a visit to a women's prison. The first time around the duchess teamed the skirt with an ivory Reiss blouse, and the second time with a black polo neck jumper – both equally fitting for a plate of smoked salmon and scrambled eggs, as well as for bundling up in a warm coat and heading off to the first church service of the day.
FYI: The Royal Family attends two services on Christmas morning. The first is low-key and private, when the Queen takes communion, and the second is when the family dress in their finery and are joined by the local community.