We have today an all volunteer military. Every single member chose to be in uniform with the understanding that it would mean being away from their family, and being stationed in places that require their presence through the Christmas season. Many of them will being working on Christmas Day. I have a lot of experience with this as I was away from home, my place of birth, for 10 Christmases. It can be a very lonely time.
Most people know someone who is in the military and serving away from. We tend to think of those serving in the war zones of Iraq and Afghanistan but we should also remember those who are serving in Germany, Korea, Japan, and many other locations including at sea. For them, it has little to do with where they are serving but the fact that they are not at home at a time when families traditionally get together. That can bring on a profound loneliness for the individual service member.
Most people know someone who is serving on active duty and is away from home right now. I can tell you from personal experience, having served 10 Christmases in the military and away from home, that one of the best Christmas presents I got was a letter from home. And that is not just from my family, but from anyone who cared to write. This is particularly true for the unmarried soldiers but of course not limited to them.
My recommendation is that if you know of such a person get his address and send him a card. Even if the only thing you can say in the card is that you are thinking of them at this time of year, that will mean more to them than you can know. Being remembered is always a wonderful thing, and it really costs you nothing more than a few minutes of your time.