solstice

Thesolstice is the time when the sunreaches its highest or lowest point at noon, resuting in the shortest and longest days of the year. (noun)

The longest day of the year is an example of the summer solstice.

See solstice in Webster's New World College Dictionary

noun

the moment or date when the sun in its apparent annual movement along the ecliptic reaches its maximum distance north or south of the celestial equator: in the Northern Hemisphere, the day of the (about June 21, marking the beginning of summer) is the time of the sun's maximum elevation and, thus, has the longest period of sunlight; the day of the (about Dec. 21, marking the beginning of winter) is the time of the sun's minimum elevation and, thus, has the shortest period of sunlight
either of the two points on the celestial sphere where the sun's path reaches its maximum distance north or south of the celestial equator

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See solstice in American Heritage Dictionary 4

(sŏlˈstĭs, sōlˈ-, sôlˈ-)

noun

Either of two times of the year when the sun is at its greatest distance from the celestial equator. The summer solstice in the Northern Hemisphere occurs about June 21, when the sun is in the zenith at the tropic of Cancer; the winter solstice occurs about December 21, when the sun is over the tropic of Capricorn. The summer solstice is the longest day of the year and the winter solstice is the shortest.
A highest point or culmination.

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