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.
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.
The reason for these changes has to do with the Earth’s yearly trip around the sun. For part of the year the Earth’s North Pole points away from the sun and part of the time toward it. This is what causes our seasons. When the North Pole points toward the sun, the sun’s rays hit the northern half of the world more directly. That means it is warmer and we have summer.
The day of the summer solstice is the longest day of the year. The length of time elapsed between sunrise and sunset on this day is a maximum for the year. In the United States, there are about 14½ hours of daylight on this day.