Why Is June 21 the Longest Day of the Year?
June 21st is the longest day of the year in the Northern Hemisphere because it is the summer solstice, which describes the tilt of the Earth's axis. This phenomenon is only observable outside of the tropics as it is the tilt of the Earth that alters the duration of light exposure of any one point on the surface.
During the summer, the Northern Hemisphere is pointed directly toward the Sun, which results in more total exposure. Alternatively, the Earth is also at its greatest distance from the Sun, so the energy is diffused over a greater distance, which somewhat reduces the overall effect. The summer solstice in the Northern Hemisphere directly corresponds to the winter solstice in the Southern Hemisphere. During this time, the Southern Hemisphere is pointed away from the sun. During an equinox, the axis of the Earth is perpendicular to the Sun, which results in nearly equal periods of day and night.
It all comes down to the 23.5 degree tilt of the Earth's axis, without which there would be no seasons and seasonal climate changes. This may be part of why the Earth has managed to develop an equilibrium of birth, death and renewal common throughout so much of the planet.