Its a tricky dilemma and one I am facing. Last year I had "christmas off" (i.e about a week) and had absolutely no trouble getting back on again - I had about a stone to go till goal and the reason I decided to eat was that I didn't want to miss all the Christmas festivities only to reach goal 2 and a half weeks later (not that reaching goal allows you to eat Christmas style, but still it seemed a shame).
If you are just starting, or a couple of months in and in the swing of it, I'd advise against it if you think you will have trouble gettting back on track, or even if you dont.
I'm back on Cambridge now (actually for the first time as I was a Lighterlife person) after having a baby and have a little baby weight to lose. Having been on it before, knowing I can get back on etc, I think I will probably eat Christmas Day only. I personally, for me, dont see the point of just having a little turkey with no gravy or anything like that - you either have a little bit of everything or nothing at all in my world! Having said that I dont want it to be a total gut fest and will probably find even a moderate meal feels like a feast after sole source.
It depends on your mindset.
I haven't totally decided but I think in all likelihood I will eat Christmas day only, then back on boxing day. It wont be a week off like last year as I have more weight to loose and am keen to get to goal, and a week of can see you several pounds up and having not lost the several pounds you would in a week so that can equate to over half a stone behind schedule.
One day off will probably see me 2 pounds behind schedule tops so I can justify that. Whilst I dont let other people govern what I do too much, it would be harsh (and bordering on obsessive in their eyes) on my in laws if I did all the cooking (and I go totally over the top, big style, everything from scratch and all the trimmings) and sat there drinking a shake and watching them eat with my beady eyes. I think I'm going to have one day of and that is that, its a compromise and getting back on track doesn't concern me too much. Unless I'm wearing a size 12 or less, my preferred day begins with a shake, continues with a bar and ends with a soup! Thats the way I like it.
x