Tonight: A Blue Moon . ... Credit Copyright: Vic Winter, ICSTARS Explanation: How often does a Full Moon occur twice in a single month? Exactly once in a Blue Moon . In fact, the modern usage of the term "Blue Moon" refers to the second Full Moon in a single month. ... A Blue Moon typically occurs every few years. The reason for the rarity of the Blue Moon is that the 29.53 days between full moons is just slightly shorter than the number of days in the average month. ...