I was going to write a post about each of the four Christmases during WWI. However I found that the Imperial War Museum had, unsurprisingly, got there before me, so I will instead direct you to their “Voices of the First World War: Christmas at the Front” which includes a podcast.
I will also direct you to a post by Rob Ruggenberg where he shares a poem entitled Christmas Day on the Somme, written by Leslie George Rub, an Australian soldier who died in WWI. It was found in his letters by his great-niece and was written on the front during Christmas 1916. It is both humorous, sarcastic and deeply touching