The second type of blue moon happens when there are four full moons in a season. A season generally only has three full moons. The third moon out of four is the blue moon, because it comes at an unusual time and doesn’t fit the pattern of naming moons according to where they fall in relation to the solstices and equinoxes. From 1932 to 1957, the Maine Farmer’s Almanac listed the blue moons that this convoluted seasonal rule created.