Eating cookies for a day does not change your weight significantly, but eating cookies for a long period of time may lead to malnutrition and obesity.
Cookies itself is a very common food, mainly made of flour, sugar, cream, etc., containing high energy, can be used for satiety, usually, eat a day of cookies alone does not affect weight.
But long-term eating will lead to the emergence of obesity, part of the cookies in order to increase the flavor, may add more fat, sugar calories are high, long-term eating high sugar, high fat cookies may lead to obesity.
At the same time, due to the cookies only carbohydrates, sugar, oil, etc., other nutrients such as protein, vitamins and dietary fiber content is less, long-term eating will lead to uneven nutrition, the risk of malnutrition.
So eating it on a separate day has no effect on your body, but you should not eat only cookies for a long period of time.