One day's excesses can't make you fat anymore than one day of good choices can make you trim. It's the pattern of our eating, exercise and lifestyle on a daily basis that make us the way we are. And if you want to change - then avoid fad quick fixes and instead focus on diet and lifestyle changes you can live with for a lifetime.
One birthday, Christmas, party feast won't reverse everything you've done - and life is for living and celebrating so do it. Then get back on track.
If you lapse - forgive yourself. If you have a major event where food is at it's core then that's ok enjoy it - the key is to get back to making healthy choices the very next day.