How can I not put weight back on after Atkins diet?

I'm on the Atkins diet, and have been on it for 1.2 weeks. I'm in the induction phase and have a formal in 3 weeks. The thing is, dessert and other high carb food will be served then which basically means I'll have to be finished on the diet by then. Is it alright to go to phase 2 and 3 for one week each? Or does it have to be for longer in order to maintain my weight? How can I not put the weight back on because I'm on a bit of a timeline. Please no criticism, I'm on this diet and there's no going back. I'm 18 btw.