If you are someone who has developed a habit of using food (usually sugary, processed food) in a self-destructive way, and you suffer badly from food cravings, and a preoccupation with food, then I don’t recommend treat days.
Sugar can be very addictive and for many of us, we can’t rely on willpower to eat it in moderation. Once we eat sugar, and for some people flour, we trigger a craving for more that is almost impossible to resist. If we avoid it altogether, with time, we no longer crave it. Every time we have a treat, assuming it is high in sugar, we strengthen our dependency on it.