This by an Oxford historian makes clear that "over by Christmas was never a promise.

Here is the conclusion.
"The words and actions of civilians and leaders do not suggest that expectations of peace by Christmas were widespread, and they certainly did not spur the recruiting boom of late summer 1914.

Soldiers more frequently predicted peace by Christmas, some hoping to play some part in the war and fearing a quick peace; once soldiers had seen action, they, like civilians, began to long for peace and their return home. The desire on the part of both soldiers and civilians for the war to end soon (but victoriously) and the particular appeal of Christmas as a religious and familial occasion made predictions of a return by Christmas a very desirable notion in 1914 and afterwards. What could be closer to the hearts of soldiers, and particularly the citizens-in-uniform of Britain's world war armies, than wanting to be at home for Christmas?

As part of the image of a nation unprepared for war, 'over by Christmas' is an iconic phrase that has become accepted as ubiquitous in and singular to 1914. It was neither. "
http://www.forumeerstewereldoorlog.nl/viewtopic.php?t=24650