Now I don't fall off the wagon when it comes to booze which massively limits the damage, but even if you're only eating with abandon you can undo a lot of good work in a weekend.
I'm actually in favour of having split personalities when it comes to healthy eating. It's great to be all virtuous and keep your diet nice and clean, but every now and then it's good to just eat whatever you'd most enjoy, and worrying about the sugar or fat content will dramatically diminish the pleasure you get from this. This is why I recommend clients stick to their healthy eating programmes 80% of the time and then 20% of the time eat whatever they feel like.
The problem of taking a whole weekend off from being healthy is that the percentages shift to 70% healthy, 30% unhealthy and even less if you start with dinner on Friday, or even earlier with the Friday fry up. Before you know it you're having unhealthy foods half the week.
The easiest way to deal with this is through containment - have a clear start and end time for your 'time off'. If it starts with Friday dinner then make sure it ends with Saturday dinner and eat healthily on Sunday. If a Sunday roast is part of your schedule then pick 3 meals over the weekend to have what you want and keep the rest healthy. So perhaps dinner on Friday, brunch on Saturday and Sunday lunch.
Also for your 'time off' to be worthwhile make sure you eat foods you really enjoy and don't give yourself a hard time afterwards. 20% of treats won't do you much harm and research suggests that calorie controlled weight loss diets are infact more effective if they include one treat meal a week where calories aren't restricted.
This is partly due to preventing an adaptive slow down in your metabolism to the restricted intake of calories and partly psychological - if you crave a treat food in the week but tell yourself you can have it on the weekend then your craving will likely disipate, whereas if something is permanently banned from your diet you're likely to crave it more. Either way it's important not to try and entirely suppress your Mr Hyde!