Perhaps, how about a big naval battle about a week before along with massive slaughter and anti war sentiment plus the Germans doing worse in the eastern front? Or does the pod need to be after 24 December 1914?
If not, maybe these circumstances could do the trick:
1] The Germans and British go on a destructive naval battle in 1914 and both sides suffer severe losses to their fleets, but the British get a pyrrhic victory [or mutual destruction], however the Germans are panicked enough to send one of their corps to the coastline temporarily. Or vice versa.
2] Russians coordinate their armies and invade East Prussia successfully, destroying the 8th Army instead of getting Tannenberg [for some reason before 1914] in September.
3] As a result of butterflies, the Germans are down several divisions less on the western front and the 1st army is encircled. However, the strains and logistics on the French result in themselves suffering heavier casualties and getting several troops encircled.
4] By the time the French reach the Aisne, the Germans are well prepared and inflict bigger casualties on the French. The Germans are able to advance further and capture all of Belgium.
5] The French, Belgian and British troops try to attack, but suffer heavier casualties with Belgium and the Channel ports are captured.
6] Meanwhile, the Austrians stop attacking Serbia after facing defeats or they manage to defeat Serbia and transfer the troops to the east.
7] Plus, maybe an Italy joining the Central Powers and suffering heavy casualties on the Alpine front.
Are these circumstances possible? And, will they lead to the Christmas truce worthy of peace signed among the sides?