It is a rare occurrence because its happened only four times in the last 115 years, the last being in 1982. 

A "super blood moon" only happens during a lunar eclipse and only when the moon is at its closest point to the Earth for the year. As the the Moon passes into the shadow of Earth, it takes on a deep red color due to the only sunlight reaching it being refracted through Earth’s atmosphere. 

Civilizations throughout the ages look at this celestial rarity as a sign of the apocalypse.