I like to think of dieting as a lifestyle change, not just a means of limiting calories. This means when the weekend rolls around, you need to continue your healthy lifestyle and forgo the many splurges Saturday and Sunday can offer, like brunch and extra cocktails.
New research from Washington University School of Medicine in St. Louis indicates that dieters tend to eat more calories on the weekend. Eating more calories than you expend means you slow down weight loss. I think an occasional splurge is OK, but two days of consistent splurges can make the needle on the scale stagnate, or even worse head in the wrong direction.
Going "weekend casual" doesn't mean you can let your willpower off the hook. Watch your portions of both food and alcohol. Plan ahead just like you do during the week. Don't sabotage your hard earned week of work in two days.