If you want to get big like Jay, you need to rest and recover. Rest days are essential for promoting muscle growth. Without them, you'll leave yourself more prone to injury, fatigue, and even training plateaus. It might be tempting to use that this extra time on the treadmill—fight the urge. You've spent your training days tearing down your muscle fibers, so now it's time to let them repair and re-grow.
Today, focus on your nutrition. Get those calories in and your body will have no choice but to respond by gaining mass. Don't skip meals, but also don't think "rest day" is a code word for "cheat day." Your diet may not be as strict as a fat-loss program would call for, but the proper macronutrient ratio is essential for muscle growth. Fuel your body with healthy food and you'll get the best from your muscles; you'll go harder and heavier on your training days. Feed your body crap, and your work in the gym will suffer.
Rest days are also excellent opportunities for fun. Spend time with your family, take your dog for a walk, see a movie, take a nap—or all of the above. Jay Cutler lives large by balancing bodybuilding, business, and life. He puts on his game face when he needs to, but he doesn't immerse himself so heavily in the lifestyle that he can't find his way back out again. Take your cues from Jay and enjoy each day as it comes.