That wasn't the case for most viewers of the blue moon on Friday, July 31, who likely saw nothing more than the beauty that is a typical full moon. That's because a blue moon really just means that two full moons occurred in the same calendar month. This explanation comes from a 1946 Sky & Telescope article that attempted to (incorrectly) clarify how the Maine Farmer's Almanac defined a blue moon. The incorrect definition stuck, however. But the phrase itself dates back to Shakespearian times, when "once in a blue moon" just meant something that rarely ever happened. On average, blue moons only occur every 2.7 years, according to NASA. The next one isn't until January 2018.