Complex sentence: September is the ninth month of the year in the Julian and Gregorian calendars and the third month to have the length of 30 day in the Northern Hemisphere is the seasonal equivalent of March in the Southern Hemisphere.
Split into simpler sentences: September is the ninth month of the year in the Julian and Gregorian calendars and the third month to have the length of 30 days. September in the Northern Hemisphere is the seasonal equivalent of March in the Southern Hemisphere.