The month of May is bookended by two full moons this year, creating a rare celestial event that won't be seen again until 2028. The second full moon is commonly referred to as a blue moon. It's not actually blue in color, but is technically called a calendrical or monthly blue moon. The rare astronomical phenomenon will be visible on May 31. What Is A Blue Moon? A seasonal blue moon is the third of four full moons in an astronomical season, which is dictated by solstices and equinoxes and usually has only three full moons, per the  Old Farmer’s Almanac . “The modern understanding of ‘blue moon’ only took off in the 1980s,” reports the almanac. “It was a result of a much earlier mistake printed in a 1946 issue of  Sky & Telescope magazine, and since then, the term has gone viral in the media.” The term "once in a blue moon" was sparked, though, this celestial event will take place again in 2028 - as it happens every two and a half year on average. What Makes This Blue Moon Special?…