After a mainly cool, very soggy Spring, many have been looking ahead towards the start of Summer. Summer officially begins at 1:16 PM, June 21st this year. Many ask me why the beginning of seasons is so "precise", so I thought I would give a little explanation.
The seasons all depend on the angle that the sun hits the Earth. In the instance of the Summer Solstice, or the beginning of Summer, the sun's highest angle (directly over head) is at its most northern point in the Northern Hemisphere, which is the Tropic of Cancer (23.26 degrees North latitude). This happens at exactly 1:16pm Eastern, on June 21st this year.
From this point, the suns angle will decrease until the Autumnal Equinox. This is the moment when the sun's highest angle passes directly over the Equator. The exact moment of this happening this year is 5:03am, on September 23rd.
After the Autumnal Equinox, the sun's highest angle progresses into the Southern Hemisphere. It will reach its lowest point, the Tropic of Capricorn (23.26 degrees South Latitude) at exactly 12:20am on December 22nd.
Once Winter has started, the sun's highest angle starts to progress its way towards the Equator once again. As the highest angle of the sun passes over the Equator, this is called the Vernal Equinox, or Spring Equinox. This year it occurs at 1:14am on March 20, 2012. From here we go on to summer again.
So, the important thing to remember is that equinoxes and solstices are only one specific instance that ushers in the new season. The exact time of these events changes from year to year, as well.