It took place back when he was a Brigadier General.
“A Little Christmas Story”
The story above was told by Dr. Albert C. Pierce, the Director of the Center for the Study of Professional Military Ethics at The United States Naval Academy. He was introducing General James Mattis who gave a lecture on Ethical Challenges in Contemporary Conflict in the spring of 2006. This was taken from the transcript of that lecture.”
Read it here. It’s not too long.