The song you are referring to is the famous carol by Joseph Mohr and Franz Gruber Stille Nacht - Silent Night. It was sung at Christmas in the trenches and there is a scene in the play Oh What A Lovely War depicting this. The words are:

Silent Night, Holy Night
All is calm, all is bright
Round yon virgin, Mother and child
Holy infant so tender and mild
Sleep in Heavenly Peace, Sleep in Heavenly Peace. There are 2 more verses.

Other people have written their own songs about this event.