So I Have Been On A Diet For Few Months

so i have been on a diet for few months and now that i am going to have a big party, i was wondering if i only eat unhealthy things in that one day and continue on my diet after that, does it matter what i eat in that one day? is my body going to get rid of most of the foods i'm going to eat or are they still going to make me just as fat as if i'd eat the same amount of sweets in a week or two?
please get back to me as soon as you can. thank you