Weight gain and fat gain are not the same thing.
You will gain weight every time when you bring in more energy than you burn.
This is evidenced by your scale weight increasing.
But it is not evidence of fat gain because fat gain is something that happens over a prolonged period of time rather than just one meal or one weekend of over consumption.
Knowing this is helpful to understand what is actually happening with your scale and why it is simply a measurement of short term in versus out. If you drink some water right now you will weigh more than you did before you drank it. This isn't the same as gaining fat.
Fat loss and gain don't happen in 2 or 5 days. They happen over prolonged periods of imbalance, be that deficit or surplus.
So don't freak out when you see the scale bouncing around day to day. That is your body doing what it's supposed to do.
To figure out if it's actually fat you have to measure body composition, not just body weight. That is the amount of fat and muscle you have in pounds.
Understand your body and how fat loss and gain result from continuous over consumption, not your worst weekend of the year.