As in previous years, we’ll probably see Kate in the car on her way to the Queen’s pre-Christmas luncheon. This usually takes place around the 20th of December (see this example from 2017).
It’s also likely that we’ll see Kate on Christmas Day (the 25th of December). Usually, the Royal Family stay at the Sandringham Estate and attend a service at a church on the grounds. Read our post from 2017.