Longest Night occurs on 12/21 | 1st Day of Winter in Northern Hemisphere & Summer in Southern
Earth's axis of rotation is tilted 23.5°. That makes the latitudes of +23.5° and −23.5° special. But how? You may have noticed two special lines of latitude on a globe of the world: One in the Northern Hemisphere called the Tropic of Cancer at +23.5° latitude and one in the Southern Hemisphere called the Tropic of Capricorn at − 23.5° latitude.
These are the latitudes where the Sun is directly overhead at noon once a year. In the Northern hemisphere, on the Tropic of Cancer, that is the Summer Solstice, usually June 21. In the Southern Hemisphere, on the Tropic of Capricorn, that is the Winter Solstice, usually December 21. These solstice days are the days with the most (for Summer) or fewest (for Winter) hours of sunlight during the whole year.