Summer begins in the Northern Hemisphere on June 20, 2012, at 7:09 P.M. (EDT).
Each year, the timing of the solstice depends on when the Sun reaches
its farthest point north of the equator. This occurs annually on June
20 or June 21 in North America, depending on your time zone.
The word solstice is from the Latin solstitium, from sol (sun) and stitium (to stop), reflecting the fact that the Sun appears to stop at this time (and again at the winter solstice).
In temperate regions, we notice that the Sun is higher in the sky
throughout the day, and its rays strike Earth at a more direct angle,
causing the efficient warming we call summer. In the winter, just the
opposite occurs: The Sun is at its southernmost point and is low in the
sky. Its rays hit the Northern Hemisphere at an oblique angle, creating
the feeble winter sunlight.
The Sun is directly overhead at its most northern point at
"high-noon" on the summer solstice, creating more sunlight in the
Northern Hemisphere on this day then any other. See your local Sun rise and set times—and how the day length changes!
Source The Old Farmer's Almanac