As the lunar cycle is 29.5 days, eventually this gets out of sync with our calendar where you normally have one full moon a month.
Blue moons are most commonly defined when we get an extra, 13th full moon in the year.
So the second full moon in a month becomes the blue moon.
However this blue moon comes from an alternative, more traditional definition when the lunar cycle results in any season having four, not three full moons.
So where we’d normally have three full moons each season. If there are four, the third full moon is then given the status of a blue moon.
As blue moons are not that common, it’s thought where the phrase “once in a blue moon” comes from.
