Plateau's can hit at any time. The best thing to do is just stick with it and don't give up. At some point the weight loss will start again. If a plateau happens, you could also up your exercise or you may need to re-examine what you're eating and make adjustments there. If you aren't keeping a food journal, it might be a good idea to start one.
As far as picking 1 day each week to eat whatever you want, I've seen it work for some people and for others (like me) it doesn't. If I go off plan for even 1 day, it is very hard for me to get back with it (which is why I've been yo-yo dieting for the past 5 years). I would talk myself into "just 1 day" and eat all sorts of things, and then the next day would come and I'd have the mentality "well I ate whatever yesterday and I'm probably going to gain anyway, so what's the point" and then I'd be off track for the entire week. Or I'd talk myself into "I'll get back with it on this Monday", but this Monday would turn into next Monday and next Monday would turn into the following Monday, etc. If you have the willpower to get back on track after 1 day, then it's your choice. Like I said, I've seen it work for some people. I'm not one of those people. :(
I've learned that if I plan ahead, I can get over just about any food craving I'm having. I've learned to cook a modified version of whatever it is I'm wanting. Like the pizza for instance. I know if I don't make something, I'm at a high risk to pick up the phone and dial Donatos or some other pizza joint. Then after I eat it, I'll feel guilty and talk myself into a million other things I've been "wanting" and spend the evening or possibly even the rest of the week just eating a bunch of crap like ice cream, candy bars, fast food, etc that I've been doing so well at avoiding. I know the cravings will pass but the guilt will stay with me every time I step on the scale.