In the United States and the rest of the northern hemisphere, the first day of the summer season is the day of the year when the Sun is farthest north (on June 20th or 21st). This day is known as the Summer Solstice.
The declination of the Sun on the Summer Solstice is known as the tropic of cancer (23° 27'). In the southern hemisphere, winter and summer solstices are exchanged so that the Summer Solstice is the day on which the Sun is farthest south.
A common misconception is that the earth is further from the sun in winter than in summer. Actually, the Earth is closest to the sun in December which is winter in the Northern hemisphere.
As the Earth travels around the Sun in its orbit, the north-south position of the Sun changes over the course of the year because of the changing orientation of the Earth's tilted rotation axes. 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.