I have a friend who has been low carb if not keto for some time and tracks his weight much more frequently than I do. He noted as I did that at the beginning of the journey he lost a lot of weight quickly (like 10 pounds in a week followed by about 0.3 pounds per day for the next few months… he shows me the graphs). Anyway he completely fell off the wagon for a week and gained 10 pounds. He is certain it will fall off just as quickly once he gets back in the program (“after turkey day”). This is important because I find that holiday challenging and if I could drop it as fast as I gain it for a brief excursion I would be sorely tempted. I am not so sure but I am sure someone on the forum has a more informed opinion than my somewhat skeptical one. Thanks in advance.
Quick weight loss/ gain question
I think your only hope is with Bikman’s theory of ketone wasting. Ramp up ketosis however you prefer, about half of the fat that gets metabolized will be converted to ketones. The ketones in turn will be wasted via urine (acetoacetate) and breath (acetone) in significant amounts. You can use the so-called ‘pee sticks’ to estimate wasted acetoacetate and a Ketonix the wasted acetone. Hope this helps. 
If he lost close to 10 pounds water weight the first time, he probably will do that again, it’s quite logical *(and too simple). I think it’s not true if you come from very different diets. I don’t lose all my water weight when I go into/out of ketosis, it’s more gradual, it seems. And our bodyweight is subject to change anyway, it’s not the same loss every time.
I went off keto a lot of time and my water weight changes happened always the same but my weight is pretty stable, it seems, as long as my carb intake is similar. That’s why I see my tiny 4 lbs water weight changes at all…
But one can never know, we are very complex systems, so many factors control our weight… It’s normal to lose significant water as we know so let’s hope it will be a similarly big weight drop. And then we will see. Very few things are certain.
I vary 4 pounds a day so I think my water variation has to be at least that. And I also lost like 20 pounds in 5 days so at least going into keto I lost that much water weight.
I think scale weight is the wrong measure.
Pre turkey tape measure belly circumference in a fasted state.
Post turkey same.
Then KCKO.
What I do is have targeted meals around the holidays. I try for it not to be a day, and I try not eat things that will provoke a complete loss of control. (I don’t know I have any of that after 6 years, though.) I’m still quite restrictive, but will eat some carbs (homemade rolls, mashed potatoes made by my mother, some ice cream pie, etc.).
I also usually fast multiple days afterwards.
I’ve been at this almost 15 months. For me if I go off plan for a few days I pick up 7-10 lbs. If I go off for one meal nothing significant usually happens weight wise, gastric upset can be an issue though. When I gained 7-10 lbs. it would come off in a week or ten days. Less if some fasting was involved. So I stay with a single meal or maybe a day for a holiday like Christmas. When I do a whole day I still try to keep it low and enjoy smaller portions of carbs and focus on the more KETO options on holidays. I don’t pig out on sweets all day. You have to find a healthy psychological balance for yourself. Totally strict if you need it, flexible for special occasions if you can handle it, but I would never suggest starting early and enjoying a holiday carb season lasting several days around a holiday.
It’s strange that you have that big of water fluctuations @Fracmeister this far along. Do you cycle heavy carb days in regularly? 
TLDR: Your friend’s guess matches my own n=1 experience.
My rule of thumb is that going into and out of keto for extended period of time results in a water loss/gain of around ~5% of current bodyweight. When I’m doing an extended fast while doing keto (84hrs), my weight generally flucuates and additional +/- 0.5% per day.
So a 250# guy would likely drop/gain around 12# in water/glycogen going into/out of keto. Once in ketosis, if there is a multi-day fast (3 day for example) involved, would lose and additional 3.75# going into the fast, and gain back 3.75# coming out of the fast. That’s my n=1 at any rate.
Four pounds or weight fluctuation in a day is nothing. Easily accomplished by water.intake alone. Ten pounds after eating carbs is probably Glycogen as well. Depending on the person’s size. After.quite a bit of CKD, I figured I carried around 14 lbs of glycogen, if I didn’t gain that I had failed to totally recompensate.
With a n=1 experiment, I could gain as much as 22 lbs of water and Glycogen in 24 hrs if I did it intentionally, all leaving within a few days.
If people who eat carbs, gain water weight, but find they appear to hang on to a few more pounds than when they started, it’s because of muscular glycogen, which requires exercise to deplete, rather than liver glycogen that goes away fairly quickly.
10 pounds is 4.5kg, I can put that on in two days if I “carb” load, because my body just HATES carbs and holds water so easily.
Though if I fast for 3 days, it’s gone.
Some people have the ability to put it on and take it off just as quick. Others have weight creep.
He would have lost a lot of weight in the beginning due to water loss, and then his weight loss would have slowed down due to the fact that his body had begun adapting, in my opinion.