You don't have to be perfect to be good and there are lots of ways to be more moderate but still have fun over the festive season.
That said, IVF takes a lot of out you, if don't have it in you to put yourself through it then take a little time - I wish I could say take as much time as you need but you know as well as I do that isn't realistic.
I really don't think that's a reason to delay unless you want to anyway. In my experience, the only way to get through the long haul of ivf is to live as normally as possible during the in-between bits. Otherwise it completely takes over. And it's hard to see that the odd drink etc in the preceding months would make any difference to anything. Without too much excess, I'd suggest doing whatever you generally do to have a relaxing and fun Christmas, and don't worry about the treatment until it happens. Sorry you're going through this, fertility treatment is utter misery xx.
I think I had alcohol during stims too (ie a very small glass of wine here and there). I think I'd definitely go ahead with the ivf but in the meantime make healthy choices as much as possible, e.g. Limit drinking to say 3 drinks a week and perhaps a couple of ciggies on Saturday night if you're struggling to give up completely. But you don't need to be all perfect for things to work.