I agree with Starry_em, the day off plan isn't usually a problem, it's finding the enthusiasm to get back on plan the following day. This happened around Christmas and I regained 3.5lbs in 2 weeks because I just couldn't get back on plan. It then took a good 2 months to get back into it properly so I try to stay on plan as much as I can.
In terms of weight gains after having a day off plan, as long as I do manage to get back on the wagon straight away I usually see a maintain or a small gain of about half a pound (though I might have a smaller loss if I reduce my syns for the rest of the week). Usually though I don't plan my days off plan. I just have slips, usually into a bottle of wine, and make bad choices after that. I've never though that I'd have a day off plan just to binge eat for the heck of it, I'm in it to lose weight and binging would be going against everything I've worked so hard for up to that point.