In the Northern Hemisphere occurs on about June 21 of each year. The summer solstice is the longest day of the year and marks the beginning of the summer season. If the earth's axis were not tilted in relation to the plane of earth's orbit around the sun (the ecliptic) there would be no seasonal changes on earth. But because the earth's axis is tilted in relationship to its plane of orbit around the sun (~23.5 degrees) and assumes a constant direction--with the North Pole pointing towards the North Star, seasons occur, and daylight hours can fluctuate in length. Each day prior to the summer solstice the sun appears farther north in the sky; but at the summer solstice, the sun reaches its most northerly position in the sky, directly above the Tropic of Cancer, 23.5 degrees north latitude. Following the summer solstice the sun moves farther south in the sky, and on about September 23 it is directly overhead at the equator (the fall equinox).
[Isis; v77; 103-4; 1986.] [The Physics Teacher; v31; 508-9; 1993.]
Source: Atmospheric Chemistry Glossary