A blue moon occurs when there are two full moons within a single calendar month. The Moon's cycle is approximately 29.5 days, which is slightly shorter than most calendar months, allowing for the possibility of two full moons in one month. Typically, this happens when a full moon occurs very early in the month (on the 1st or 2nd), followed by another full moon at the end of the same month (on the 30th or 31st). This event happens roughly every two to three years. There is also a less common definition of a blue moon that refers to the third full moon in a season when there are four full moons instead of the usual three. Despite the name, a blue moon does not actually appear blue; the term simply denotes the unusual timing of the full moons. Occasionally, atmospheric conditions caused by particles from fires or volcanic eruptions can make the Moon appear bluish, but this is unrelated to the calendrical blue moon. In summary, a blue moon is mainly a calendrical event that happens due to the mismatch between the lunar cycle and the calendar month length, resulting in an extra full moon in the month about every two to three years.