I think it basically works like this--when you are eating lightly during the day, your body uses what you eat for energy, and any deficit is made up by burning reserves.
Then, when you eat more than usual at that party, you eat more than your body needs at the time, so the excess gets stored as reserves.
So, as long as you don't go TOO low during the day (like fasting) or TOO high at the party, it all balances out.
If you've been eating on plan for some time, you'll find that your capacity to overeat has also lessened, so you may not WANT to pig out at the party. And if you pace yourself--eat slowly--avoid seconds, thirds--then you can still be having a good time and enjoying what you have.
Be free from the extremes! Don't fast all day and then stuff yourself--that's a recipe for loss of control and subsequent weight gain.
Just my opinion!
Jay