What is a Blue Moon?
A blue moon is a rare ocurrence, and today’s (July 31) is one. The next blue moon will happen on 2018.
Based on NASA’s definition of a Blue moon it is when there are two full moons in a single calendar month. The truth is, that’s it! I guess it’s not so special after all, it won’t appear blue either.
But there are several occurences that the moon did appear blue and it happens after volcanic eruptions. The moon later may also appear red.
Often, when the Moon is hanging low, it looks red for the same reason that sunsets are red … The atmosphere is full of aerosols much smaller than the ones injected by volcanoes. These aerosols scatter blue light, while leaving the red behind. – NASA