A blue moon happens about every two to three years, according to NASA. There are two definitions: a monthly blue moon is the second full moon in a single calendar month, which can occur because the lunar cycle (about 29.5 days) is slightly shorter than most months, and a seasonal blue moon is the third full moon in an astronomical season that has four full moons instead of the usual three. Counting both kinds, only about 6 percent of full moons are blue moons, and despite the name, a blue moon does not look blue.
Source: NASA Science, “Super Blue Moons: Your Questions Answered”
Image: Full Moon (photo by Gregory H. Revera, CC BY-SA 3.0), via Wikimedia Commons.