A blue moon happens primarily due to two reasons:

1. The common calendrical meaning of a blue moon is when there is a second full moon within a single calendar month. This happens because the lunar cycle (about 29.5 days between full moons) is slightly shorter than most months, allowing for the occasional occurrence of two full moons in one month. This type of blue moon occurs roughly every 2 to 3 years and is a predictable event with no special astronomical significance.
2. The moon can also appear visually blue in rare cases due to atmospheric conditions. When particles such as smoke, dust, or ash from large forest fires or volcanic eruptions are present in the atmosphere, they can scatter red light and cause the moon to take on a blue or bluish tint. This phenomenon is unpredictable and rare.

Thus, the term "blue moon" most often refers to the second full moon in a month, but the actual blue-colored moon is an uncommon visual effect caused by specific atmospheric particles. The phrase "once in a blue moon" comes from how rare the visual blue moon is, although the calendrical blue moon is more common.