Well you want to do it on a weight day because contrary to belief, the nutritional value of your cheat meal is extremely high. That is obviously if you aren't just gorging down sweets/cakes. If you eating shyt with genuine nutritional value and packs a punch calorie-wise like burgers or similar, you are getting the best out of that meal. Now only on a weight day are you breaking down your muscles so you want that protein from the meal to recover and grow the muscle you broke down. That leads into the timing. If you eat that meal prior to the workout, you aren't capitalising off the meal's value to recover your muscle because you haven't broken it down yet.
Plus if OP was to get the workout in early that day and then spend the day eating cheat meals, and he's someone who usually adheres to a strict diet, his metabolism will be vastly improved. You need to shock your system once in a while.
Hopefully this clarifies it.