It usually happens just once every couple of years
The second full moon of July rose Friday night, a phenomenon referred to as a “blue moon.”
Though the phrase “once in a blue moon” is meant to describe a rare event, a blue moon is neither rare, or blue. “Blue moon” is actually a term used to describe the second full moon in a single calendar month. Most years only have 12 moons, but this year has 13, which happens every two to three years, according to astronomers. “Blue moons” don’t actually appear blue, but the moon can appear blue after events on earth like wildfires and volcanic eruptions, according to NASA.