Summer solstice happens precisely when the Earth's axial tilt is most inclined towards the sun, at the degree of 23° 26'. 

Summer solstice in 2011 was celebrated by many countries in the Northern Hemisphere.  There, it marks the longest day of the year, the shortest night of the year, and what many consider to be the beginning of summer.

Summer solstice occurs on the longest day and the shortest night of the year in the northern hemisphere

Winter solstice occurs on the shortest day and longest night of the year. December 22 is the date for Winter solstice this year.

Come December, it will be winter solstice in the Northern Hemisphere and summer solstice in the Southern Hemisphere.

The two solstices of the year happen when the earth's tilt towards the sun reaches an ultimate point. In the June solstice, the Northern Hemisphere is tilted towards the earth while the Southern Hemisphere is tilted away from the earth.