"on Christmas Eve, 24 December 1914, [...] German troops began decorating the area around their trenches in the region of Ypres, Belgium, for Christmas. They began by placing candles on trees, then continued the celebration by singing Christmas carols, most notably Stille Nacht (Silent Night). The British troops in the trenches across from them responded by singing English carols.
The two sides continued by shouting Christmas greetings to each other. Soon thereafter, there were calls for visits across the "No Man's Land" where small gifts were exchanged whisky, jam, cigars, chocolate, and the like. The soldiers exchanged gifts, sometimes addresses, and drank together. The artillery in the region fell silent that night. The truce also allowed a breathing spell where recently-fallen soldiers could be brought back behind their lines by burial parties. Proper burials took place as soldiers from both sides mourned the dead together and paid their respects.
The truce spread to other areas of the lines, and there are many stories of football matches between the opposing forces.
In many sectors, the truce lasted through Christmas night, but in some areas, it continued until New Year's Day."[link]