If I stick to my diet all the time but one Saturday every month I decide to eat my favourite takeaway, doner, chips and cheese, how will this affect me?
You will have an enjoyable Saturday but may well find you don't lose as much that week. As to whether you go up, down, or stay the same long term will depend on your overall intake and output. Count the calories, do the sums.
Is that one day a week, one day a month or just one day full stop ? think about it, you are proposing a higher intake of food which is bound to affect the scheme of things, as Old_Geezer suggests.
Nothing wrong with that if it helps you stick to your diet the other 27 days of the month.
You can help yourself by eating very sensibly the rest of the day and reducing the portion size.
Bet it gives you indigestion, though, if you've been cutting down on fatty food.
Depending on what your "good" diet is, you may see a gain but it will most likely be water weight. It will take a few weeks of bad food choices to gain actual fat. Be careful though with your splurge...it can trigger cravings. Do you want that?