Also, get as much light-both sunlight and artificial light-as you can. Take advantage of the daylight hours with a walk. The light on your retina will get that signal sent to the photosensitive parts of the brain to say, "Hey, it's not as bad as it seems..." And use bright lights inside. The brighter the better.
Exercise is helpful too, but try to do it during the daylight hours, or in a well-lit place. If you get your exercise in the morning or during the day, you will give your body a consistent signal that it's not hibernation time.
Keep your sleep habits regular, and don't sleep too much. That hibernation tendency is self-perpetuating. Keep your sleep hours down to eight to nine hours a day; no more than that.
People tend to drink more during the holidays. How does alcohol affect mood?
It has a euphorian effect while the blood alcohol level is rising, but that is only going to last as long as the blood alcohol level is rising. Which means that within fifteen to thirty minutes after you finish consuming that last drink, your blood alcohol level is falling, at which point you're going to start feeling bad. So again, it's best to drink in moderation.
What can you suggest to people who are feeling sad during the holidays about a person who is absent, or perhaps has passed away?
Sadness is a signal. It's a reminder that you had a relationship that was really important to you, otherwise you wouldn't feel sad about it. There may be contained within those sad feelings some regret about things that didn't go well or anger about something that didn't go right, and this can complicate these feelings.