Be careful. One key to a healthy lifestyle is moderation, and moderation means setting limits, applying portion control, and making choices based on long-term health goals, not immediate gratification. If you know that you have a family picnic (i.e. unhealthy food fest) coming up, do your best to maintain your healthy eating and exercise habits in the days prior to it. Go on an extra walk or make an extra trip the gym. Make sure that you eat a balanced breakfast the day of the event, and consider eating a healthy meal before you arrive so that your hunger won't tempt you to overindulge. It’s okay to enjoy yourself and to celebrate important events in your friends’ lives, as well as your own. Make your friends and experiences the center of these occasions—not the food.
When you're "on a diet" excuses like these make it easy to go off of it. After all, you just go back on again once you're done having your fun. Forget the "diets" and start going on a "healthy lifestyle" instead.