Weight gain has very little to do with fat gain or even loss.
If you had a big eating day then it would be understandable to most that the weight gain would be from the weight of all the food consumed still being digested and in storage (stomach and bowels).
Most weight gain however is from water storage within organs and muscles from carbohydrate intake.
An Extreme 250+g Carb day on your diet would cause a minimum 1kg gain with the water storage in your muscles and organs alone, plus water weight not stored and all the food waste.
It is not uncommon to gain over 2kg of body weight after an Extreme Carb day however fat storage can be nil and in most cases you have actually lost body fat even during all that eating.
You just can't see it on the scales yet. Wait a few days and then your true weight will be shown.
Generally the more food you eat on the specified days (so long as it is the right types as noted) the more fat you will lose by the end of the week.
Never judge fat loss on your body weight throughout the week until Friday/Saturday morning when we prepare your body for the weekend by emptying muscle and organ storage to show your true weight each week.
Other factors that are likely to cause a weight gain are -
Alcohol or insufficient metabolism boosting carbohydrates the day before.
Alcohol slows down metabolism while eating the wrong foods can cause a good overnight weight drop followed by a weight gain during the day due to a poor metabolism.
Your body does most of the calculations while you sleep so it is important to consume most of your calories at night so they do not get utilised as energy during the day when we want to use stored body fat.
Sleep obviously then also affects metabolism, too little or too much.
An example of weight gain would be -
Friday you eat as per your plan except adding a few glasses of alcohol.
Even if the alcohol option is a low carb, it is the alcohol itself that affects metabolism not the carbs.
Saturday morning you still get a good weight drop however it is this day that will be affected, regardless of how well you eat.
Sunday morning you weigh yourself and see a bigger weight gain than expected despite your good eating on the Saturday (previous day).
This can not be avoided while drinking alcohol but you can obviously try and catch up during the rest of the week by limiting alcohol and by keeping with your food tests recommendations and following you plan.
Obviously the less alcohol consumed while dieting to reach your goal weight, the better.
It is also wise to avoid fats on the day of the alcohol consumption and ensure that you still consume your carbohydrate quota, hopefully at least an hour before the alcohol is consumed.