I see many people fast after a feast. However, from the concept of autophagy (e.g. Valter Longo), shouldn’t a fast be done before a feast?
Or it does not matter?
I ask this because I hope for Christmas I will have reached my target weight, and I want to be able to handle family food. When should I fast? Before or after Christmas?
I think this depends on your goals.
If you simply want weight loss / keeping weight down through the holidays, then fast before Christmas.
If you are successful with your fast - you can feast with a little more gusto (you put some weight loss “in the bank” and can make a withdrawal).
If you are not successful with your holiday-pre-fast - you know you need to stay a bit more careful at the holiday table.
If, on the other hand, you were looking for strength gains by letting autophagy rip away at your body while fasting and then work out like crazy during your refeed (to replace and add new muscle, leveraging all the extra growth hormone your body is producing). Then - it might be best to fast after Christmas because you are unlikely to want to get into heavy workouts during the holidays but might be more likely to want to push hard as part of some New Year’s resolution.
Read this and make a decision. https://idmprogram.com/feasts-and-fasts-the-cycle-of-life-fasting-part-11/
I find holidays are easiest if I just eat the things that are traditional that fit my WOE, lots of veggies and meat. That way I don’t have to go through questions etc.
Personally I find feasting (in this case a tad more than usual like 10% more carb and a 10% more fat) before an extended fast makes the fasting easy. I eat what would be an everyday menu on the day after the fast. You might need to play around and find out what works best for you. Remember stress and holidays have tons of that, is a factor as well. Do not disregard its effects on your hormones, appetite, and mental attitude. All the best sorting yourself out.
I usually try to limit what I eat that’s off keto to a certain number of meals. And then at some point, I’ll get in a longer fast of a few days, starting, say, January 2nd. 
Since i omad…my daily meal is my feast. So i fast before. Eat. Fast after when i am done. Repeat.
Alternate your feasting and your fasting. In other words, fast before and after feasting, and feast before and after fasting.
I have to say that I pretty much eat what’s served at Thanksgiving. But I’ve been doing this a long time, and I don’t have trouble getting back on plan and also limiting the damage.
I find ‘after’ is more targeted. You are asking the question because you are assuming you will put on some pounds while you feast with your family. If you fast after the feast it’s a simple matter to continue whatever fasting protocol you are using until you have re-reached your target weight.
I do this all the time. I don’t exactly fast (I do daily 24 hour ‘intermittent fasting’ but I have always considered that to be time restricted eating and not fasting. For me fasting begins at 36 hrs), but I do a modified and more restrictive diet whenever I put on a few pounds. This can happen fairly easily if I eat a bit of extra carbs with family or sometimes after I’ve had a sweet treat or two.
I don’t calorie restrict. I time restrict (one meal a day) and I restrict palatability. For me what works best is cutting absolutely everything except fatty cuts of beef and salt. Nothing else. When I do that I will lose weight every single day until I surpass my goal weight. A salted ribeye steak is delicious but you will put it down when you are full.
Anyhow, that’s my protocol because I find fasting too hard, but it’s still probably preferable for most people to fast (faster weight loss anyhow).
But you have no experience fasting. You can’t easily go from eating all day on your current plan to fasting. Everyone here who fasts is first keto and fat adapted and then generally works up their fasting ‘muscle’. That takes time and effort. There is as reason why everyone is fat today. If just giving up food until you got thin again was easy everyone would be thin. You need to be both keto and to practice to take fasting out of the ‘very difficult’ range.
Almost everyone here only eats once or twice a day already. That’s the power of the diet that perhaps you don’t understand. A low carb/high fat diet keeps your hunger and satiety hormones under amazing regulation. You need to eat much less frequently and this allows your insulin to drop and your body fat to be released as fuel. If you go back to a higher carb diet after your weight loss however, you won’t be enjoying the benefits of this regulation and that’s what makes fasting so much easier for keto people.