Christmas does not have to be stressful, shopping does not have to be stressful, being with family and in-laws does not have to be stressful, NO EVENT has to be stressful! It is the emotion we attach to the event that makes things seem stressful. Before the event we start thinking about "what ifs" and usually it is about something going wrong.
You may have the person at the party who gets drunk and it is harder to control but you can even do little things to control it. Make weaker drinks, get low alcohol beers/wine, and have a designated driver arranged to take them home. Or it could be the difficult relative that always starts arguments. Keep the conversation on light topics or ones you don't particularly care about, or excuse yourself to use the loo to break the state of aggression building.
TIPS: Before the event, think about it all going well. Keep your mood up with Christmas music and naming 10 things you are grateful for about the event before it happens. Focus on the positive side and remember to breath 6 times deeply to calm you back to a neutral place. Remember, thinks seem worse in our minds so ask yourself "is this really that bad" and "what can I do to improve things immediately" and let your creative side go wild. NO NOT drink excessively or allow yourself to react in negative, self destructive ways like over eating or smoking. It is not worth it and you are the only one who suffers as a result.
Enjoy the season and relax!