A cheat day is a day when you deliberately set your meal plan aside and eat what you feel like, usually well above your needs. Unlike a refeed, it is not built around carbs on purpose, it is open.
The morning after, the scale often reads one to two kilograms higher. Only a small part of that is fat.
Where the jump comes from
A cheat day is almost always a carb day. Much of the carbohydrate goes into your glycogen stores, and glycogen is stored with about three to four parts water. Add the food still sitting in your gut, and probably some water from salt. How much salt contributes exactly is less clearly established than the glycogen effect.
Example: Using the approximation of about 7,700 kcal per kilogram of body weight, a 3,000 kcal surplus adds at most about 0.4 kg of tissue. The scale often shows 1 to 2 kg more. The difference is water and gut content. These numbers are a worked example.
If you go back to eating normally, most of it is usually gone after two to four days. Your trend barely moves in that time.
When it does count
For your weekly balance, a cheat day can be bigger than it feels. If you hold a 500 kcal deficit on five days of a cut, that is 2,500 kcal, and one day with a 3,000 kcal surplus wipes out the whole week's deficit. You will not see that the day after, but in your trend after two to three weeks.
After a cheat day, simply eat normally again, without compensatory fasting or trying to "flush water", and keep weighing yourself every day.