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.

source:http://www.calendar-updates.com/info/holidays/us/summer.aspx