A second full moon in a calendar month is commonly called a blue moon, but that is not the astronomical definition. Astronomers refer to it as the third full moon in a season with four full moons, since most seasons only have three full moons. And don't let the name throw you through a loop - this moon doesn't look blue.
The phrase "once in a blue moon" refers to something that is rare, and a blue moon is decidedly rare. This seasonal moon occurs on May 18, and two full moons that occur in the same calendar month will not happen again until October 2020, when the full moons will begin and end the month.
However, blue moons have not been rare recently. There were two full moons in January 2018 and March 2018, making this a blue moon year. A doubling of the blue moons of the calendar month will not occur again until 2037, and will also occur in January and March.
And, if you want to go ahead and mark your calendars even later, TimeAndDate.com has a few other blue moon combinations for you to consider.
“Between 1550 and 2650 there are 20 years that have a seasonal and a monthly blue moon. The next time is in 2048 while the previous one was in 1934. Triple blue moons, a combination of a seasonal and two monthly blue moons in the same calendar year, occur 21 times in the same time interval. The next one is in 2143, while the last time was in 1961. "
Okay, so someone reading this is likely close to that triple blue moon in 2143, but maybe we discovered cryogenic freezing or putting our consciousness into robots before then.