Although summer solstice marks the beginning of summer, it is not the hottest time of the year and Earth is not the only planet to have a summer solstice. For example, on Uranus, each summer solstice lasts for 42 years. Mars’ solstice occurs a few days after earth’s June solstice.
The summer solstice occurs when the tilt of a planet’s semi-axis, in either northern or southern hemispheres, is most inclined toward the star that it orbits. This happens twice each year (once in each hemisphere), at which times the Sun reaches its highest position in the sky as seen from the north or the south pole.
The summer solstice occurs during a hemisphere’s summer. This is the northern solstice in the northern hemisphere and the southern solstice in the southern hemisphere. Depending on the shift of the calendar, the summer solstice occurs some time between June 20 and June 22 in the northern hemisphere and between December 20 and December 23 each year in the southern hemisphere. The same dates in the opposite hemisphere are referred to as the winter solstice.
When on a geographic pole, the Sun reaches its greatest height, the moment of solstice, it can be noon only along that longitude which at that moment lies in the direction of the Sun from the pole. For other longitudes, it is not noon. Noon has either passed or has yet to come. Hence the notion of a solstice day is useful. The term is colloquially used like midsummer to refer to the day on which solstice occurs. The summer solstice day has the longest period of daylight – except in the polar regions, where daylight is continuous, from a few days to six months around the summer solstice.
Worldwide, interpretation of the event has varied among cultures, but most recognize the event in some way with holidays, festivals, and rituals around that time with themes of religion or fertility.
Mindful that the solstices and equinoxes symbolize the fertility of the land, agricultural and food production systems, cultural heritage and their millenary traditions; the General Assembly of the United Nations acknowledged that the celebration of those events is an embodiment of the unity of the cultural heritage and centuries-long traditions, and further play a significant role in strengthening the ties among peoples on the basis of mutual respect and the ideals of peace and good-neighborliness. It, therefore, recognized 21 June as the International Day of the Celebration of the Solstice.
Source: wikipedia.org | un.org
In 2016 Summer Solstice (International Day of the Celebration of the Solstice) in USA falls on June 20.