It is common in ‘diets’ that are low cal or low fat to have a cheat day, this is because they are very restrictive but ultimately still rely on a high carbohydrate fueling system and adjusting total calories per day. The failure in this approach is that you don’t teach your body to burn fat as an energy source, and ultimately never gain control over your weight.
In the first few weeks of going keto, it can take a few days to get back into keto if you switch back to carbs. In this time all fat burning and fat adaption process is halted, so one cheat day per week could mean you are only in Keto 2 or 3 days per week!
It’s not all doom and gloom! As you become more fat adapted and make fat burning your default ‘setting’. You will return to Keto quicker if you push the carbs.