A blue moon happens when there are two full moons in the same calendar month. Since the lunar cycle is about 29.5 days, and most months are longer than that, occasionally a full moon will occur very early in a month and another at the end of the same month, producing a "blue moon." This is a relatively rare event, occurring roughly once every 2 to 3 years. The term does not mean the moon is actually blue in color. There is also a second definition called a "seasonal blue moon," which occurs when there are four full moons in one astronomical season instead of the usual three. Rarely, the moon can physically appear blue due to atmospheric particles from events like volcanic eruptions or large forest fires that scatter red light and cause the moon to look blue, but this is unrelated to the calendar definition of a blue moon. In summary, a blue moon is mostly a calendrical phenomenon caused by timing and the mismatch between the lunar cycle and the calendar month, not a change in the moon's color.