June 20th, 2012, which officially marks the beginning of summer in the Northern Hemisphere, is also the longest day of the year in this hemisphere, with most of the regions experiencing roughly around 14 hours of daylight. In the Southern Hemisphere, though, it will be the shortest day of the year; and mark the beginning of winter. For the countries lying to the south of Equator, December 21, 2012, will be the longest day. Even though it is quite a common phenomenon, not many people are aware of the fact that there is something like the longest and the shortest day of the year. Technically put, a 'day' is the period of 24 hours, wherein the Earth completes a single rotation. Colloquially, however, the term is used to refer to the period between the sunrise and sunset, when it is bright outside. (In contrast, the term 'night' is used to refer to the period between the sunset and sunrise, when it is dark outside.) On June 20th, 2012, the sunrise is scheduled for 05:25 and sunset for 20:31; which amounts to 15 hours and 05 minutes of daylight. The same was 9 hours and 18 minutes for January 1, 2012, (sunrise - 07:20 and sunset - 16:39), and will be 9 hours 28 minutes for December 1, 2012 (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). Summer Solstice and the Longest Day of the Year Basically, days and nights are caused due to the rotation of the Earth, with the side facing the Sun experiencing day and other side experiencing night. As the Earth is tilted on its axis at an angle of 23° 26', at one point it aligns in such a position that it is either inclined towards or away from the Sun. Such alignment, wherein the apparent position of the Sun is either perpendicular to the Tropic of Cancer or the Tropic of Capricorn, is referred to as solstice. It takes place twice in a year(;) once when the tilt of the Earth's axis is inclined towards the Sun, and once when the tilt is inclined away from the Sun. 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? The alignment happens gradually over the course of time, and hence we witness the duration of the day increasing as we close in on June, and decreasing thereafter. On June 21st, the length of the day is at its peak, which again differs place to place. Though rarely, this phenomenon, which usually occurs on June 21st, can also occur on June 20th or June 22nd. (This change in the solstice dates can be attributed to the fact that we refer to the Gregorian calendar wherein every fourth year is a leap year.) In 2008, it occurred on 20th June; which was only the second time since 1975 when it didn't occur on June 21st. In 1975, it occurred on 22nd June, and that will happen again only in 2203. 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.