What is a Blue Moon? While the definition of what is a Blue Moon has changed over the years, the popular theory is that whenever two full Moons appear in a single month, the second is christened a “Blue Moon.” Due to our calendar days, the appearance of a Blue Moon is actually a relatively rare occurrence as one rises on average only once every 2 1/2 to 3 years. Most years have twelve full moons that occur approximately monthly, but sometimes the days of the month are less than the cycle of the moon, which is 29.5 days. This can cause the moon to rise twice in one month (the second being the “Blue Moon”), near the first and the last days of the same month. This relatively rare occurrence has spawned the saying “once in a blue moon”.