In the northern hemisphere, the longest day of the year when the Sun is farthest north. In the southern hemisphere, winter and summer solstices are exchanged. The summer solstice marks the first day of the season of summer. The declination of the Sun on the (northern) summer solstice is known as the tropic of cancer (23° 27′).
http://scienceworld.wolfram.com/astronomy/SummerSolstice.html
