Your weight from day to day doesn't really tell you anything - you can't possibly gain or lose 2 pounds of fat overnight! The only thing that number tells you is your water weight. Alcohol acts as a diuretic, so it's likely that weight loss is because of the beer. And as Jo said, carbs can be addicting, so you might not even see the negative effects of your carb day until the rest of the week, when you find it harder to turn down the next carby treat.

I don't think a militant LC diet is necessary for everyone, but... you say in your post that you've already experimented with a more moderate LC diet without seeing weight loss.