The second full Moon this month happens tomorrow (Friday July 31). NASA says whenever you get two full moons in a month, the second one is now commonly referred to as a “Blue Moon”. The first full Moon this month was on July 2nd. NASA says there are roughly 29-and-a-half days between full Moons, so on average, we see one about every two-and-a-half years. The next Blue Moon isn’t until January 2018.