To find the date of the next blue moon, consult a blue moon calendar such as the ones at MoonGiant.com or EarthSky.org. Since the term "blue moon" refers to the second full moon in a single month, you can also determine when the next blue moon takes place by using any calendar that includes the dates of full moons, such as the moon phase calendar at MoonConnection.com.
Blue moon calendars typically list all the dates of blue moons in the near future and recent past. The blue moon calendar at MoonGiant.com, for example, lists the dates of all the blue moons occurring from 2012 to 2042. To find the date of the next blue moon, simply find the next date listed on the calendar.
To use a conventional moon phase calendar, such as the one at MoonConnection.com, look for the next month that contains two full moons, as a "blue moon" refers to the second full moon within a single calendar month. Because the length of the lunar phase cycle is so close to the length of the calendar month, a blue moon is a relatively rare occurrence. Blue moons always occur at the end of the month, usually on the 31st day.