If you're already losing 2-3 lbs per week, then my math says you should be 20 lbs lighter in 2 months...so, I'm not sure why you're asking. However, you don't need a fad diet. I would suggest low carb, low calorie (no lower than 1,200) and low sodium. Plus daily exercise...preferably running & weight lifting (since these would be most effective), but substitute other forms of exercise if you can't run or don't have access to weights. If you need to shift a few more pounds at the end of the 2 months, you can always go for a body wrap the day before (go as late as possible). The weight will come back in the next few days, but it might help you to fit in your dress.
Oh, and once you've dropped the 20 lbs, don't forget to get back to the healthy this and that...long-term they're the only things that count.