"It's cool now and the evening has closed around you to become full night. The night for once is comforting and you take a coke and some peanut butter and jelly and go up on the roof of the cabin whit your tape recorder and sit for a while, quietly, watching flares float silently through the sky and flashes announce disquieting intent somewhere in the distance. You call down to one of your men and ask him to draft a message to the Admiral in Command of all Naval Forces in Vietnam and also to the Commander of Market Time. IT says "Merry Christmas from the most inland Market Time unit." You hope that they'll court marshal you or something because that would make sense. But the night soothes everything and the people and things that are close to you dart through the mind and bring the only warmth and peace that there is. Visions of sugar plums really do dance through your head and you think of stockings and snow and roast chestnuts and fires with birch logs and all that is good and warm and real. It's Christmas Eve."
http://www.boston.com/globe/nation/packages/kerry/journal_day2.shtml
Kerry arrived in Vietnam on December 1, 1968. The Swift boats only began to engage in combat operations (according to reports) two weeks later, so around December 14th.
Notice that Kerry was hoping to be courtmartialed on December 24th, just ten days later.