The Christmas Truce explained

 The Christmas Truce is a great event of 1914.

In the Christmas Eve of 1914, while World War I was ongoing, the Germans stopped the fire and then got out their trenches, to wish the English "Merry Christmas". They sang carols and exchanged gifts and cakes.

"I was in the trenches Christmas Eve day and Boxing day. The Germans had all along their trenches Christmas trees and big fires all alight, making a pretty sight. They were singing carols at the top of their voices, and so were we, when daylight came, a German soldier came over to our trenches and say, 'English soldier, may I come near, do not shoot me,' so we said 'No.' He said that the German soldiers wished the English soldiers a merry Christmas. We wished them the same. Then we went and shok hands with some of them, exchanged cigarettes and little things." 

-Letter from an English Soldier

There are more letters saying that the English asked Germans if they're tired of war, and they said 'Yea!...Something like that..!', or that they played football in the neutral zone during the truce.

Unfortunately, governments banned information about these things. It also never took place again, because the British high command dreaded that homogeneous event could undercut confidence and erode the rivalry among German and British troops.

In the wake of the events of the Christmas Truce, there was a repression by the military higher-ups of the both forces. They tried everything possible to ensure that it'll never happen again.
In my opinion, it's really sad that the rivals can't unite at least for Christmas, like they did in the magical Christmas of 1914.

The Christmas truce mainly happened between the British and German because the land they were fighting for meant nothing to them. Ypres is Belgian, not German nor British.