“Once in a Blue Moon” might sound like a long time, but we’re about to experience one this weekend – and it will be your last time to catch one until August 22, 2021.

The full moon on Saturday, May 18, 2019, will be a Blue Moon, according to NASA. There are two main definitions of a Blue Moon, both referring to an additional full moon that appears within a certain window of time, either a season or a month. Unfortunately, neither involves a sudden color change from our loyal cosmic companion.

This upcoming moon falls under the seasonal Blue Moon definition, which refers to the third full moon in a season that has four full moons.

The other definition, which is arguably more widely known, denotes the second full moon in a calendar month. It's easy to assume that a full moon occurs once a month – as in a “moon-th” – but occasionally a calendar month will fit in two full moons. This is because each cycle of the Moon's phases lasts about 29.5 days, yet there are 30 or 31 days in a calendar month (except February). So, every two or three years, there will be 13 full moons, not 12.

While the monthly definition might be more popular, some claim it’s actually the result of a publishing screw-up. The definition can be linked back to the March 1946 edition of Sky & Telescope magazine when a reporter misinterpreted the term from an old issue of the Maine Farmers' Almanac. Not realizing the Moon's phases in the book were based on a seasonal calendar, he understandably assumed the “extra” moon referred to the months. This definition was then used as an answer on radio games and a 1980s Trivial Pursuit board game, and so the misinterpretation spread.

As mentioned, the Moon won't literally turn a shade of blue. That said, it's believed the saying "Once in a Blue Moon" actually originated in 1883 after the eruption of Krakatoa threw so much dust into the atmosphere the Moon took on a blue hue. 

The traditional name for May's full moon is sometimes the Flower Moon, used to signify the flowers that bloom during this month in the Northern Hemisphere. Traditional names of moons are often a mash-up of terms used by ancient cultures, Native American tribes, and European pre-Christian pagans. Nowadays, they don't signify all that much, although they do make for some pretty incredible names. For example, in January 2019 we got to witness the "Super Blood Wolf Moon".