If your kids are old enough, talk to them about how they would like to do Christmas within the schedule you and the other parent have determined. Over time, it does get better. But while things are fresh and hard, make sure you take care of yourself first.
I was sad from Nov - Mar, but I was not clinically depressed. I was doing all the healthy things I could do to get my energy and spirit back on the right track in my life. And while I was not immediately joyous as a result of these efforts, I learned that by sticking with the program, my program of healthy and conscious living, I could have all the things I wanted in my life.
Depression is truly a soul killer. And it might be a slippery slope towards suicide if we don't get a handle on it, both personally and as a society.