blue moon
extra full moon that appears in a subdivision of a year: either the third of four full moons in a season, or a second full moon in a month of the common calendar
en-US/ˈblu ˈmun/
en-GB/ˈbluː ˈmuːn/
extra full moon that appears in a subdivision of a year: either the third of four full moons in a season, or a second full moon in a month of the common calendar