Basically as an adult you have a daily calorie limit which will be around about 1800-2000 calories a day for a woman (you can find thing online that give you a guesstimate of how many calories you need per day)as long as you dont over eat on these calories then you should maintain. If you know your going for a big blow out meal then save calories have a smaller brekkie and lunch on the day and if necessary a smaller brekkie and lunch the day after and you should be ok. Eat healthy choices keep the fat content down and then you can live your life without being ruled 100% by what you eat though it should always be close to the forefront of your mind. it takes 3500 extra calories to put on a pound but thats only an extra 500 calories a day so if you over eat by this then you can stick on a pound a week!!
You are in total control of what goes in you mouth no one else can make you eat stuff you dont want or thats bad for you its down to you and your choices if you maintain.. or so i keep getting told by loads of people!!