Two major events in George Washington's military career, which happened seven years apart, are associated with Christmas.
Taking those two events in reverse chronological order, the second one did not occur on Christmas Day, but rather on December 23, 1783. That was his announcement, about which I wrote here, that he resigned his commission as commander of the American army, in order to return to Mount Vernon. Due to the timing, it is sometimes called his "Christmas Farewell".
As many historians see it, Washington thereby gave up the opportunity to rule as a military dictator. His action established the principle of civilian supremacy over the military.
The earlier event was his Crossing of the Delaware on December 25, 1776. That set up a surprise attack on the British forces' mercenaries from Hesse, who were encamped at Trenton, New Jersey.
Washington's forces crossed over from Pennsylvania; the Delaware River forms the boundary between Pennsylvania and New Jersey.
Most of us associate that event with a painting by Emanuel Gottlieb Leutze, that is currently in the collection of New York's Metropolitan Museum of Art. Of course, as has been said over and over, the image in the painting cannot be historically accurate. Had they struck such a pose, the boat would have capsized, sending our hero and his soldiers into the icy river. But the heroic pose has cemented this image of the event into our collective national memory.
The victory at Trenton is credited with keeping the American cause alive, at a critical time in the early days of the Revolution.
PBS's website has a page that sets up the background of the battle. And here, a local historical society adds a further description.