A Blue Moon has two definitions which are in use today.
I find them both easy to work with.
The first (and the more commonly known) definition is when a single calendar month has two Full Moons (when the Moon is illuminated 100%) - the second Full Moon is called a Blue Moon. This occurs every ~33 months.
The next of these Blue Moons is set to occur on January 31, 2018.
The second (and actually it has been around much longer than the other yet is less known) definition is when a season (a 3 month period) has 4 Full Moons instead of the expected 3. In this case, the 3rd Full Moon is called a Blue Moon. This is the traditional definition of a Blue Moon, and it occurs much less frequently.
The next "traditional" Blue Moon will be on May 18, 2019.