Definition of blue moon in English:

blue moon

noun

  • 1A phenomenon whereby the moon appears bluish owing to smoke or dust particles in the atmosphere:

    ‘people saw blue moons for years after the 1883 eruption of Krakatoa’
  • 2A second full moon in a calendar month:

    ‘tomorrow's full moon is a blue moon’
    [as modifier] ‘it can be no coincidence that July is a blue moon month’