One day can't make you fat!
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.