"All over by christmas" Source?

I am afraid that I may not be of much help.

You may have seen this. It is from "A Surgeon's Journey" by James Johnston who joined the RAMC in 1914

'Next day I made for the War Office and was directed to A.M.D.1, if I remember aright. This I found was a small office; and, to my surprise, sitting at the desk was one of my old house-surgeons in a captain's uniform. We greeted each other warmly, "What can I do for you, sir?" he said." I want to join up," I answered, cheerfully. He looked at me with obvious embarrassment. "Well, sir, that's really rather difficult. You see, sir, you're a bit old for us. You are over thirty, and we really only want young newly qualified men to do dressings in the front line. We don't need Fellows of the College. You're too heavy guns for us. The war will be over by Christmas."

However, this obviously isn't the origin. By this stage, the expression seems to have been commonplace.

I came across the following quote:

"We are beginning to collaborate with the English, they are real gentlemen, here as everywhere, and I think that we shall be able to do a good job with them and be home by Christmas”

(General Maxime Weygand, French Army).

But I have not been able to establish a date or place for this.

Now, the Kaiser did say in a speech to troops that they would be "home before the leaves fall" (August 1914).