When Friday's blue moon arrives, don't expect it to be blue—a blue moon isn't actually that color, reports CNN, though some full moons can indeed have a bluish hue. The phrase "once in a blue moon" refers to something that is rare, and it was once used this way in the Maine Farmers' Almanac when describing the third full moon in the rare season that has four (typically, there is only one full moon a month, thus a three-month season will have three full moons). But in 1946, Sky & Telescope magazine published an article that misunderstood this definition, instead calling the second full moon in a calendar month a blue moon. And it is indeed fairly rare: It happens once every 2.7 years or so.
This has become the modern definition and is used to describe Friday's full moon, which is the second this month (the first was on July 2); it'll be the last of its kind until January 2018. Interestingly, while the most recent blue moon according to this modern definition occurred at the end of August of 2012, the most recent blue moon according to the original definition occurred more recently, in August of 2013, when the full moon was the third of four that summer. As for the blue moon on July 31, while it may seem to last all night, it's technically an "instantaneous event" that occurs at 6:43am EDT on the nose, reports Space.com. (Earlier this month, a man took an incredible picture of the International Space Station passing in front of the moon.)