A plan definitely helps--and it doesn't have to be a tight, formal plan with every ounce in place, but just some idea that you can follow once you get to dinner.
For example, you could plan portion and serving control: Decide what your favorite foods are, go to a calorie site and look up what a "serving" of the food is and how many calories are in it. Then, use this information to plan what you'll be eating at Easter dinner. For example, you could restrict yourself to a large slice (or 6 oz.) of ham, a small scoop of scalloped potatoes, some salad with the amount based on how much dressing and what kind is on it. You could skip the dinner roll, and have a small sliver of pie. PERIOD. No seconds, no mindless eating. Skip the appetizers, or portion them accordingly. If alcohol is involved, set a limit on the number of drinks you have. Two is plenty for most people.
See what I mean? These are just sample foods--you can look up your own.
And, keep in mind that one day of eating over your calories is not going to make you permanently gain back all your weight lost, although the next morning your weigh in won't be fun due to water retention...
Jay