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 the year. While the days are longer than nights in summer, exactly opposite happens in winter. The entire phenomenon is attributed to 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 this alignment of the Sun and Earth to which we owe both, the longest and the shortest day in a calendar year.
Winter Solstice - The Shortest Day of the Year
Solstice occurs twice in a calendar year: first, when the apparent position of the Sun is at its northernmost limit (i.e., the Tropic of Cancer/23.5°N), and again when the apparent position of the Sun is at its southernmost limit (i.e., the Tropic of Capricorn/23.5°S). When the Sun is at its northernmost limit at 23.5°N, it is referred to as the summer solstice, and when it is at its southernmost limit at 23.5°S, it is referred to as the winter solstice.
Similarly, when the Sun's apparent position is at the equator, which happens twice a year, it is referred to as equinox.
The Northern Hemisphere-Southern Hemisphere Difference
In the Southern Hemisphere, the summer solstice occurs when the Sun is at its southernmost limit at 23.5°S and the winter solstice occurs when the Sun is at its northernmost limit at 23.5°N. Owing to the differences in the 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 its northernmost limit, the Northern Hemisphere experiences more daylight than the Southern Hemisphere.
If you happen to be a resident of the Northern Hemisphere―the chances of which are more, you will experience the longest day when the Sun is at the Tropic of Cancer, and the shortest, when it is at the Tropic of Capricorn. If you happen to be a resident of any 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 June 20 or 21 (sometimes on June 19 or 22) and at the Tropic of Capricorn on December 21 or 22 (sometimes on December 20 or 23). In accordance with this, the longest day of the year has to be June 20 or 21 for the Northern Hemisphere, but December 21 or 22 in the Southern Hemisphere. Similarly, the shortest day of the year has to be December 21 or 22 for the Northern Hemisphere, but June 20 or 21 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). Similarly, when the Sun is positioned at the 23.5°S latitude the North Pole and surrounding areas experience 24 hours of darkness, while the South Pole and surrounding areas receive 24 hours of daylight.