I think many of us have this problem, to some extent.
Since you've identified it, look upon it as a challenge to overcome.
Stock up on Friday evenings on healthy, low-cal foods: fruit, veggies, rice cakes. Get an air-popper and make air-popped popcorn. It's virtually calorie-free (although not very tasty). You could snack on it 24/7 and not gain any weight.
When you're tempted to snack on weekends, exercise instead.
Clean your house, go out for a walk, or call some friends and hang out; do some sit-ups, or put on music and dance in your living room.
Do something creative: write a story, draw a picture. Read a good book.
Drink some water, coffee, or ice tea.
Just think: if eating more on weekends can make you gain weight, what might eating less
on weekends do?
Weekends are a good time to eat less than normal, because we usually don't have much to do, so we don't expend as much energy.
Imagine how much more pleasant Mondays will be, if you're feeling rested, refreshed, and thinner, rather than bloated and sluggish and stuffed.
Experiment with how little you can eat on weekends and still feel okay (no headaches, no stomach pains, etc). You might be surprised with how little you can get by on, when you're just hanging around the house not doing anything in particular.
Give it a try for a few weekends. Consider it a test of willpower.
If you're miserable, you can always go back to your old eating habits.