On the first night of 2018 you might look up at the sky and say, "Wow, the moon looks huge tonight. Well the reason why is, it is the first "supermoon" of 2018. A supermoon happens when the moon becomes full on the same day it reaches the point in the moons elliptical orbit closest to the earth.
Saying something only happens “once in a blue moon” typically means it’s extremely rare. True blue moons — defined as the second full moon in a month — aren’t actually that uncommon, but if you don’t catch the one on Aug. 31, it’ll be three years before you get another chance.