Leg day is by far the most important of all the gym days. Your legs are your foundation. Your entire body is built on top of them. If you plan on strengthening your upper body, it only makes sense to also strengthen the part of your body thats responsible for holding up your upper body, right? And besides, if you stay committed to leg day, you may end up being able to do some pretty amazing things, like crushing watermelons with your thighs.