The phrase describes anything that happens very rarely. You might say a childhood friend calls once in a blue moon, or that a truly original idea comes along once in a blue moon. It is one of the most common ways in English to express rarity without specifying a precise frequency.
A "blue moon" originally referred to the third full moon in an astronomical season containing four full moons. On average, this happens roughly every 2.5 years. The popular modern definition — the second full moon in a single calendar month — is widespread but stems from a 1946 misinterpretation published in Sky and Telescope magazine. In either case, the event is genuinely uncommon, which is where the phrase draws its meaning.
The earliest known reference to a blue moon as something rare appears in a 1528 pamphlet by William Barlow, where it is used to mean an impossibility. By the 19th century the phrase was in common use across Britain and America for any infrequent event. Interestingly, the Moon can literally appear blue after volcanic eruptions or large wildfires, when fine ash particles in the atmosphere scatter red light — but this rare atmospheric phenomenon likely contributed very little to the phrase's origin.
Using the traditional astronomical definition, a blue moon (the third full moon in a season with four) occurs roughly every 2.5 years. Using the popular modern definition (the second full moon in a calendar month), it occurs about every 2 to 3 years.
Yes, rarely. After major volcanic eruptions or large wildfires, fine ash particles can scatter red wavelengths and make the Moon appear blue or purple to observers. This is a real but genuinely uncommon sight.