My German isn’t very good but here’s my translation. 

In the upper left hand corner: “Christmas Eve in the trenches”.

Below the picture: “To our brave soldiers in enemy territory heartfelt Christmas greetings and God’s protection and blessing in the New Year”.

From the collection of The Western Front Association

James (“Jim”) Patton BS BA MPA is a retired state official living in Shawnee, Kansas and a frequent contributor to several WW1 e-publications, including "Roads to the Great War," "St. Mihiel Tripwire," "Over the Top" and "Medicine in the First World War." He has spent many hours walking the WW1 battlefields, and is also an authority on British regiments and a collector of their badges. An Army Engineer during the Vietnam War, he does work for the US World War 1 Centennial Commission and has memberships in the WW1 Historical Association, the Western Front Association, the Indian Military Historical Society and the Salonika Campaign Society.