It's fine as long as the rest of the week is healthy, then it's good to have a cheat meal so you don't feel deprived through the week. It actually helps prevent binging.
No, one day won't make you gain 1.5kg. You need to eat 3500 EXCESSE (extra) calories to gain just .5 or a kg, so 10,500 calories to gain 1.5kg of fat. I doubt you ate that much, so no worries, and good luck!