You must have realized that the length of a day - as based on
daylight (i.e. the period between sunrise and sunset) varies over the
course of a year. While the days are longer than nights in summer,
exactly opposite happens in the winter. The entire phenomenon is
attributed an astronomical event known as solstice, which is dependent
on the seeming movement of the Sun between the Tropic of Cancer and
Tropic of Capricorn. It is the alignment of the Sun and Earth, as a
result of which we experience the longest and the shortest day in a
calendar year.
Solstice occurs twice in a calendar year - first when the apparent position of the Sun is at its northernmost limit (Tropic of Cancer/23.5°N) and again, when the apparent position of the Sun is at its southernmost limit (Tropic of Capricorn/23.5°S) This event of astronomy is further categorized into two parts - summer solstice, which is the longest day of the year, and winter solstice, which is the shortest day. When the Sun is at its northernmost limit at 23.5°North, it is referred to as summer solstice, and when it is at its southernmost limit at 23.5°South, it is referred to as winter solstice. Similarly, when the Sun's apparent position is at the equator, it is referred to as equinox - which occurs twice a year. This, however, only applies to the northern hemisphere of the Earth.
The Northern Hemisphere-Southern Hemisphere Difference
In southern hemisphere, summer solstice occurs when the Sun is at its southernmost limit at 23.5°South and winter solstice occurs when the Sun is at its northernmost limit at 23.5°North. Owing to the differences in apparent position of the Sun, each of the two hemispheres experience summer solstice and winter solstice at different times of the year. When the Sun is positioned at the northernmost limit, the northern hemisphere experiences more daylight as compared to southern hemisphere. Contrary to this, when the Sun is at its southernmost extreme the southern hemisphere experiences more of daylight as compared to its northern counterpart. If you happen to be a resident of northern hemisphere - the chances of which are more, you will experience the longest day when the Sun is at Tropic of Cancer, and shortest when it is at Tropic of Capricorn.
If you happen to be a resident of some country in the southern hemisphere, you will experience the longest day when the Sun is at the Tropic of Capricorn and shortest day when it is at the Tropic of Cancer. Generally, the Sun is at the Tropic of Cancer on 20th or 21st of June (sometimes on 19th or 22nd June) and at the Tropic of Capricorn on 21st or 22nd of December (sometimes on 20th or 22rd December). Going by these dates, the longest day of the year has to be 20th or 21st of June for the northern hemisphere, but 21st or 22nd of December for the southern hemisphere. Similarly, the shortest day of the year has to be 21st or 22nd of December for northern hemisphere, but 20th or 21st of June for the southern hemisphere.
Interestingly, when the Sun is positioned at the 23.5°N latitude, the region around the North pole experiences 24 hours of daylight (day) while the region around the South pole experiences 24 hours of darkness (night) at a stretch. Similarly, when the Sun is positioned at the 23.5°S latitude the North pole and surrounding areas experience 24 hours darkness while the South pole and surrounding areas receive 24 hours of broad daylight.
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