Weight from one week of not exercising

Will I gain weight from one week of not exercising and eating fatty foods?

well, yes.

your body will start to lose it's lean muscle that you have been developing from the exercise and healthy food...and if you eat more calories than you burn, then you're going to gain weight.

I would be surprized if you gained more than five pounds though.

yes but it wont stick especially if its fatty foods im assuming that your trying to gain weight to turn into muscle. buy a protein that has a mass gainer in it or just increase how much you eat but keep working out. also split your routine in half. if your doing biceps and you have a 2 hour routine do everything but half the amount of sets so it should only take an hour then wait at least 5 hours and do the last hour doing this will on average double the size of your muscles so you'll gain weight but dont do it for more than 4 months because the muscle cant keep up tearing so often.