Not much about life in the trenches of World War I is very festive. What is festive-ish is that in 1914, an unofficial Christmas Truce was spontaneously declared in many trenches, as British and German troops decided to put their differences aside for a bit.
Bizarre things happened, including unarmed soldiers venturing into No Man’s Land (the space between the trenches), exchanges of gifts (apparently mostly food and cigarettes), games of soccer, and even caroling. Here’s an eight-minute film documenting the Christmas Truce through animation and a letter from a British soldier:
(Video via Brain Pickings.)