On June 20th, 2016, the sunrise is scheduled for 05:47 and sunset for 19:59; which amounts to 14 hours and 12 minutes of daylight. The same was 9 hours and 18 minutes for January 1, 2016, (sunrise – 07:20 and sunset – 16:39), and will be 9 hours 28 minutes for December 1, 2016 (sunrise – 07:01 and sunset – 16:29). As you see, the length of a day increases and decreases over the course of a calendar year. Starting from January, it continues to increase till June 21st (at times June 20th or 22nd.) From this particular day, it starts decreasing, and continues to decrease till December 21st (at times December 20th or 22nd).
This phenomenon and the apparent position of the Sun in the sky enables the sunrays to reach the northern or southern extreme, which, in turn, determines the duration of a day. When the Sun is at the Tropic of Cancer, Northern Hemisphere receives more daylight, and therefore has longer days and shorter nights. When it is at the Tropic of Capricorn, things work the other way round, and the Southern Hemisphere experiences daylight for a longer period. Simply put, the hemisphere of the planet which is inclined towards the Sun experiences more daylight, and hence the days here are long and nights short.
Basically, the day on which the summer solstice occurs, with reference to a particular hemisphere, happens to be the longest day of the calendar year for that hemisphere. The length of a day will differ from one region to another depending on its latitudinal location. While the regions close to the Equator experience 12 hours of daylight, regions along the Tropics experience 14-15 hours daylight on this day. Interestingly, the places beyond 66.5°N and 66.5°S experience daylight for the entire 24 hours in summer, as the tilt of the Earth brings these areas directly under the Circle of Illumination for a part of the year. In fact, places like Canada, Greenland, Alaska and Sweden, witness a strange phenomenon referred to as the ‘midnight Sun’, wherein the Sun is visible even at midnight.
20th June or 21st June?
Though the actual astronomical event is restricted to a few seconds, the day on which this event occurs is informally referred to as the ‘summer solstice’. Being the longest day of the year, it also has a great significance in various cultures, where it is celebrated in the form of rituals, festivals and even gatherings, such as midsummer parties; all meant to welcome the Sun and the new season.