The Christmas Truce
During World War I (then simply "The Great War"), trench warfare led to brutal attacks, slow battles of attrition, and horrors such as poison gas and barbed wire. Wikipedia sums up a bit of the reality of the trenches like this:
The Germans introduced poison gas; it soon became used by both sides, though it never proved decisive in winning a battle. Its effects were brutal, causing slow and painful death, and poison gas became one of the most-feared and best-remembered horrors of the war. Commanders on both sides failed to develop tactics for breaching entrenched positions without heavy casualties.
You can read more about life in the trenches, though it's not festive. What is festive-ish is that in 1914, during the week leading up to Christmas, 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:
(Via Brain Pickings.)