The term "blue moon" traditionally referred to an extra moon in a season: if a season had four full moons (rather than the more common three), then the third of the four moons was known as a blue moon. Jul 31, 2015 – 10:46 (GMT), second full moon in July Saturday, May 21, 2016 at 9:15 PM (third Full Moon in a season) Wednesday, January 31, 2018 at 1:26 PM (second Full Moon in single calendar mon…