The truce didn't occur all along the line, mainly in the Flanders sector held by the British. Here is what Historian David Stevenson had to say on the matter

In the midst of this carnage occurred one of the wards most poignant moments, the 1914 Christmas Truce. On 24 December lighted Christmas trees appeared in the German trenches in Flanders and both sides began to sing carols. On Christmas morning British and German soldiers met in no man’s land, chatted, smoked, play football, posed for photographs, and buried their dead. Often the ceasefire lingered for several days until it was ended (with apologies by the units on the ground) at the insistence of the high commands, which ensured that at Christmas in subsequent years it happened much less, if at all. The episode seems to encapsulate the lack of rancour between many frontline soldiers, who – now the heady opening days had receded – found themselves trapped in a killing machine by pressure from above. Unofficial truces and tacit agreements to moderate the violence would continue to characterise the Western front during 1915, in the French sector (where the Christmas truce was less prevalent) as well as the British.

Though it did occur in French sectors, it was far less common, mostly due to the animosity felt by the French towards the Germans who were after all occupying a significant part of their country. Max Hastings presents some interesting insights as well:

Along several sectors of the Western front, a singing competition developed between rival trenches. The German second guards division, for instance, saying ‘Stille Nacht’ and ‘O du Froliche’, and hoisted a Christmas tree on their parapet. When the French had made their own coral contribution, the Germans answered with ‘Vom Himmel hoch.’ Then the contest became more nationalistic: the French bellowed the Marseillaise, the Germans ‘Wacht am Rhein’ and ‘Deutschland uber alles’ before giving three cheers for the Kaiser.

Alexander Johnston wrote laconically: ‘my first and I hope my last Christmas on active service.’ Near Ypres, Wilbert Spencer ‘saw about nine or 10 lights along the German lines. These I said were Christmas trees and I happened to be right… On Christmas day we heard the words “Happy Christmas” being called out, whereupon we wrote up on a board “Gluckliches Weihnachten” and stuck it up. There was no firing, so by degrees each side began gradually showing more of themselves and then two of them came halfway over and called out for an officer. I went out and found that they were willing to have an armistice for four hours and carry our dead men halfway for us to bury – a few days previous we had had an attack with many losses. This I arranged and then – well you could never imagine such a thing. Both sides came out and met in the middle, shook hands, wished each other the compliments of the season and had a chat.’

Men of the French 99th infantry regiment, which had similar experiences, were affronted to find the truce shattered by heavy German fire on New Year’s Day. The following morning a Bavarian lieutenant came over to explain apologetically that his superiors had taken fright about the malign impact of fraternisation on the serious business of winning the war. A German regimental report described another such incident near Biaches, in the Somme sector.

1914-1918: The History of the First World War by David Stevenson

Catastophe: Europe Goes to War in 1914 by Max Hastings