What is a blue moon?
There are two definitions of a blue moon. Space.com says one arises from a misunderstanding of the other. The older term references the third full moon in a given season that has four. In general, there's one full moon per month (once every 29.5 days). So, when there are four in three months, an extra full moon has popped up. This is called a seasonal blue moon, and it takes place about once every two-and-a-half years, according to NASA.
The newer definition is when there's a second full moon in a single month. With a full moon taking place about once every 29.5 days, a calendar month with two is pretty rare. You might say it only happens once in a blue moon.
Both are perfectly fine ways to use the term. Space.com cites astronomer Donald W. Olson, who said in Sky & Telescope, "With two decades of popular usage behind it, the second-full-moon-in-a-month (mis)interpretation is like a genie that can't be forced back into its bottle. But that's not necessarily a bad thing."
Since neither blue moon is blue in color, you might ask what you will see. It's just a full moon. It's a rare occurrence, but it won't look substantially different than other full moons. A blue moon will not necessarily be a supermoon, which appears larger and brighter than an average full moon. The one occurring on May 18 comes after a string of three straight supermoons earlier this year and those are the last of them until 2020.