Great news, AtlantaRose! 15kg is an excellent achievement. I reckon relaxing your diet on weekends is fine. Life is for living and enjoying, so sharing dinner with friends or enjoying a party is not 'bad' or 'cheating'. Sticking to a diet means being consistent, though, and if you have pizza and wine on Sunday night, just make sure that you're vigilant with your food/drink for the rest of the week.
Congrats again, on your weight loss.