Groove if you're anything like me, I would weigh this week, draw the line, take the gain, and start 2017 on the right foot.
There are two ways you can handle this. You can call the entire week until WI a write-off. It's Christmas, things aren't normal, you will get back on plan next week. OR you can say "right, Boxing Day's over" and get back on plan tomorrow morning, 100%, having SP days if you feel you can handle them, writing everything down, weighing your HEs etc, and seeing how much damage limitation you can do.
I gained 6.5 lbs over Christmas week last year (I went for option 1, and it was lovely, but I couldn't do it every week). It's probably going to be similar this year. I am 90% cheese and chocolate right now.
If I hadn't gone to the WI between Christmas and New Year last year, I would've carried on eating like it was going out of fashion, and wouldn't have gone the week after New Year (because I would've been embarrassed) and then I wouldn't have gone the week after and so on. And then I wouldn't have gone at all, and I would've gained back all I had lost (because I've been there and done that before) and then I wouldn't have got to target.