June 21 will be the longest day of 2011, at least for those of us in the Northern Hemisphere.
At exactly 1:16 p.m. EST, the Summer Solstice will occur, at which point the sun will reach its highest point in the sky and stay there briefly.
In the Southern Hemisphere, June 21 marks the Winter Solstice, the shortest day of the year.
The summer solstice signifies "Midsummer," the day on which we receive the most hours of daylight all year.
However, according to NASA, the event is actually associated with the beginning of the summer season. This is because the oceans, which significantly effect the climate, have yet to warm back up by June 21 after the cold temperatures of winter.
The term solstice originates from the Latin words sol (sun) and sistere (to stand still). At this point in time, the Earth's northernmost point at the North Pole tilts maximally toward the Sun.
In other words, the sun reaches its northernmost spot in the sky before it starts to appear to move southward. Days will become shorter and nights longer until the winter solstice on December 22.