When I say, "Not losing your shit over the holidays" I am referring to not completely becoming a fat ass by stuffing your face with food and also not being consistent with your training.  I am not referring to losing your shit on your in-laws.  I have cool in-laws so I have no advice in that department.

If you aren't in a prep and you struggle to get your training sessions in at this time of the year I recommend doing one of two things:

1. Cut back on frequency.  If you normally train 4 or 5 days a week, cut it back to 3 sessions per week.  This will allow you more flexibility to get some fun stuff done during the holidays with less stress or burden to get all of your workouts in.
2. Increase training frequency by training more days.  You might think wtf? but I have personally found that when I struggle to stay focused, I do much better if I increase the demands for training vs. decreasing them.  This tends to keep ME more focused and structured.  Everyone is different.

If you do feel that kicking back is your best option, have a little fun with the workouts by experimenting with different exercises that you normally wouldn't do.  Play with different machines, different angles, play with different ... anything.  I have found that when I experiment, I usually can come away from it with at least one or two things that I might continue to use in my regular training that I otherwise may not have tried.  I will give one example:  While on vacation I worked out on the cruise ship and the horizontal leg press was broken.  It would work only in about the top 1/2 of the range before bottoming out.  I found out that using this limited range of motion was far more productive than I thought it would be as I was able to handle much more weight and still pump the hell out of my quads without putting my lower back in a vulnerable position due to my chronic lower back issues.  When I returned home and was back to training 100% the following week, I kept the partial rep range for leg presses in my training plan and it has been paying off very well.  I would never have played with this option if I wasn't forced to, based on a piece of broken equipment.

You could also do Crossfit to stay motivated.
Just kidding.  Don't get that stupid.

When it comes to your diet, you again have two main options:

1. Stay tight as hell with your diet and take Tupperware to family gatherings and look like a douche.

2.  Enjoy regular food that your parents or grandparents made for the holiday without metaphorically slapping them in the face and acting as if you are better than the food they made.

If you opt for option 2 (and you should if you aren't in prep mode), you can offset the added calories you are going to eat at these holiday functions by doing something like a keto-type diet on the days that you don't have family functions.  If keto or lower-calorie dieting isn't your thing during the holidays, add some cardio to offset those calories.  That way, you can enjoy the holiday food without the guilt or thinking you are going to turn into Fatty McFatty after eating one big meal.

Something I like to do (and did on vacation, as well) is to go very low or even no carb but enjoy a lot of different foods while being aware of portion size.   I was able to hold my condition for 10 days while on vacation by simply limiting carbs as much as possible and still enjoyed some really good food.  I allowed myself to get satiated on primarily proteins and fats.  Even if you do have some carbs, limit them to a smaller percentage of your meals and eat the carbs at the end of the meal, after the protein and fats.  That way, you are less likely to overeat on carbs.

If you drink alcohol, stay clear of high-calorie drinks like egg nog and go more for mixed drinks that contain alcohol and a diet cola.  You will get less calories having these types of mixed drinks than you will drinking things like Kalua, flavored rums and beer.

Keep in mind that going completely off track and just enjoying the holidays with family and friends is also a viable option.  When I used to compete a lot, I would take most of the time off between Thanksgiving and Christmas and just kind of rest my brain, knowing I was balls out the other 11 months out of the year.  The only problem with doing that is you usually have a mess to clean up for the first month of the new year.  If you are good with that and your brain can handle it (most of us struggle with this), go for it.  Just know that the first of the year will eventually arrive and you don't want to regret overindulging at the holidays.

No matter what you decide, I hope you enjoy family and friends at the holidays.  You do not have to eat like a hog to enjoy family and friends.  However, it is a lot of fun to take a break and be normal for a few days, too.