I will also edit my answer once you post your food logs, but this is actually pretty common and there is a pretty clear and well-studied conclusion here. Obviously, I can't 100% be sure since I haven't seen your diet, but you say your cheat day is on Friday and that over the weekend you gain weight. Well...this is pretty expected, happens to me all the time during contest prep.
What's happening is your manipulating your glycogen stores via carbohydrates or excess calories during a single day. Think of your glycogen stores like little balloons that hold water (more accurately, promote water delivery and holding onto them), except instead of air blowing up the balloon, in this case it is carbohydrates and in some cases if you don't eat much carbs, it is due to excess calories from other sources (your body can turn other macros into glycogen). Glycogen is simply just an easy access tank of energy that your body has access to. When you're in a caloric deficit and your carb intake isn't too high, these "balloons" start losing volume and therefore weight, but just like a regular balloon...you can still blow it back up.
It has been proven in various studies that the difference between max glycogen levels and min glycogen levels can vary up to 7lbs!!! The exact amount does depend on the individual's genetics and insulin sensitivity however.
The fact that you don't work out on saturdays also confirms/strengthens the fact that your glycogen stores will be fuller than usual and hence you will be holding more weight. Try this for an experiment, have a really hard workout on saturday (legs if you don't fear death), and weigh yourself Sunday morning (glycogen manipulation does take time), see if this is still occuring...I can almost guarantee it won't, at least to the same extent.
Moreover, note that excess sodium can also have the same effect of holding water. So if, on your cheat days, you eat a lot of fast food and processed foods (which have higher sodium than most foods), then this sodium will also make you retain more water. Combining this with glycogen manipulation, these two can lead to some large fluctuations, so just be aware. Lucky for you (and me), weight/fat loss seems to not really care about a few days, and the process usually depends on your overall weekly calories
EDIT: I just looked into this a bit more, and when I said above that glycogen manuplation does take time, I should specify that it can take upto 2-4 days depending on a lot of factors. Until that time, the combination of excess carbs and proteins will reduce release of body fluids, so once there is an impulse in these amounts, your body will get confused and release a lot of water through urine, sweat, even air through CO2, and thus this is yet another effect for fluctuating weight. This is because since you just cheated, you increased your blood sugar more than usual and thus reduced your cortisol (the most evil hormone to ever exist for muscle building) by a considerable amount, so your body stopped holding on to water for a brief time, had your glycogen stores been depleted, this wouldn't have had nearly the same effect.
