My grandfather Thomas John Williams was in the Royal Army Medical Corps in the Great War (WWI). He died before I was born. He said that on Christmas Day in 1914 the Germans came out of their trenches and crossed into no-man's land to sing Christmas Carols and play football. They called him "Tommy". Less politely his comrades called them Krauts.
This is oral history from my family, so I can't be sure that he was there or was just telling what someone else had told him, however, this is a historic event that has been well documented - see Christmas Truce