A lot of people have trouble with this, especially after weigh in, they have the day off, but then one day turns to 2 which turns to the whole weekend and then theres only 4 days on plan!
Since youre doing it at home why dont you move your weigh day to a tuesday or something? then you cant binge over the weekends? Or would you consider joining a group?
Personally I just wouldnt buy the junk food in (as I learned over christmas I dont have THAT much self control) If it was just a one off I would say dont worry, draw a line under it and carry on with the plan. But as youve mentioned its a recurring problem I would suggest doing something that takes your mind off snacking. Could you find a new hobby or do something that keeps you busy especially after WI (you could get your magazines/books/internet and plan your next week of meals). Look for SW alternatives for snacks, there are some fab existing threads for these. Plan your week meals and then use a food diary to record what you do actually have. Have you ever recorded all of the syns you have during a binge this may even shock you into stopping.
Flexisyns you add all of the syns up that you need to enjoy your meal/night even if its a whole box of chocolates, fish and chips, tube of pringles and 3 bottles of wine. thats a heck of a lot of syns lets say 300. You start with 300 and then count them off as you have them. That way if you get to the third bottle of wine and dont fancy it you dont have to drink it. The theory is counting down you will feel better if you have some left over. Where as if you count from 0 up to 300 its more like a target you are trying to reach and will force yourself to get there.
The best thing is youve identified a possible danger area and are looking for help and info.x.x.