Exactly how often is "once in a blue moon"? According to NASA, the celestial occurrence isn't especially rare: A blue moon happens about once every 2.5 years. The next blue moon will appear the night of May 18, 2019, and because the event marks the first full moon of May, it will also be a flower moon.
What Is a Blue Moon?
Instead of describing color, like a blood moon, the term blue moon is reserved for an additional full moon that appears within a certain window of time. There are two types of blue moons: monthly blue moons and seasonal blue moons. A monthly blue moon, the more popular of the two definitions, is the second full moon that occurs in a single calendar month. This usage is fairly recent, and likely originated from an error printed in a 1946 issue of Sky and Telescope magazine.
A seasonal blue moon is the older meaning, and it describes the third moon in a season that has four full moons. Each season—winter, spring, summer, and fall—typically sees three full moons, and in the rare event there are four, the third is singled out as the anomaly. This is sometimes the preferred definition of astronomy-minded people because it's based on natural equinoxes and solstices rather than the Gregorian calendar.
When to See the Blue Flower Moon
The full moon set to light up the night sky in May will be a seasonal blue moon. The time of year it occurs—May—makes it a flower moon. The first full moon of each month has a special name: A worm moon is a full moon in April, and a wolf moon is a full moon in January.
To catch 2019's blue flower moon, look up the night of May 18. The Moon will be at its fullest when it's precisely at 180° ecliptic longitude opposite the Sun—which occurs at 5:11 p.m. ET on May 18.