I think you are familiar with the story, but anyway in short: Hazel and his friends are going to find some food for the Frith feast, but got lost in the snow. On their way they encounter a Fox and Bigwig fell into the ice. Looking for warm place for Bigwig to recover they met Buttercup who invited them into their burrow, located near a human house in some kind of labyrinth. Of course, Hazel and the others assumed it is a trap like Cowslip's burrow, but in the end they found out it is a save place and celebrating Christmas eve together with the humans, before they returning home.
Although this two episodes contributes not much to the overall story, I still like it very much. For instance we see Watership Down covered in snow and also the other landscapes are beautiful drawn. And the Watership Down rabbits learn that not all humans are elil.
It's of course true when you say, this is produced to fill the gaps and bound some connection with Christmas, but I think they did a good job. They also produced a tune especially for this two episodes called "Winter on Watership Down".