It is an extra Full Moon in a calendar
A blue moon usually means the second Full Moon within one calendar month. The lunar phase cycle lasts about 29.5 days, while most calendar months last 30 or 31 days. Occasionally a Full Moon occurs near the beginning of a month, leaving enough time for another before the month ends.
An older seasonal definition also remains in use: the third Full Moon in an astronomical season containing four Full Moons instead of the usual three. These definitions describe calendar placement, not color, and they do not always identify the same date.
Does the Moon turn blue?
No. A blue moon normally looks like any other Full Moon. Its brightness, rise direction, apparent size, and atmospheric color depend on ordinary geometry and observing conditions.
Rare atmospheric events can make the Moon look bluish when smoke or volcanic particles preferentially scatter red light, but that optical effect is unrelated to the calendar term. A calendar blue moon can look orange near the horizon, and a genuinely blue-looking Moon need not be a blue moon.
Why it is uncommon
Twelve lunar cycles take about 354 days, roughly 11 days shorter than a solar calendar year. Those extra days accumulate, producing a thirteenth Full Moon every few years. The familiar phrase “once in a blue moon” reflects that relative rarity, although the event is predictable rather than mysterious.
