During World War I, German and British troops stopped fighting and celebrated Christmas together.

At Christmas time in 1914 – during World War I – German and British troops got out of their trenches, refused to fight, and celebrated the holiday together.  Besides refusing to fight, they also shared music, singing, chocolate, alcoholic beverages, photos from their families back home, and so forth.

John McCutcheon wrote and performed a great song about this: https://www.youtube.com/watch?v=sJi41RWaTCs

Also, I encourage you to watch this 21-minute video:  https://www.youtube.com/watch?v=kYwKD2dc33A&feature=youtu.be