In the northern hemisphere, the longest day of the year (near June 22) 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′).
The summer solstice is the longest day of the year, respectively, in the sense that the length of time elapsed between sunrise and sunset on this day is a maximum for the year. Of course, daylight saving time means that the first Sunday in April has 23 hours and the last Sunday in October has 25 hours, but these human meddlings with the calendar and do not correspond to the actual number of daylight hours. In Chicago, there are 15:02 hours of daylight on the summer solstice of June 21, 1999.
The above plots show how the date of the summer solstice shifts through the Gregorian calendar according to the insertion of leap years. The table below gives the universal time of the summer solstice. To convert to U. S. Eastern daylight saving time, subtract 4 hours, so the summer solstice occurs on June 21, 1998 at 10:00 a.m. EDT; June 21, 1999 at 15:47 (3:46 p.m.) EDT; and June 20, 2000 at 21:36 (9:36 p.m.) EDT.
Note that the times below were calculated using SummerSolstice in the Mathematica application package Scientific Astronomer, which is accurate to within only an hour or so, and in practice gives times that differ by up to 15 minutes from those computed by the U.S. Naval Observatory (which computes June 21, 1999 at 19:49 UT instead of 19:47 UT and June 21, 2000 at 01:48 UT instead of 01:36).
Here is a QuickTime movie illustrating the tilt of the Earth’s equatorial plane relative to the Sun which is responsible for the seasons. The dates of maximum tilt of the Earth’s equator correspond to the summer solstice and winter solstice, and the dates of zero tilt to the vernal equinox and autumnal equinox.