"The longest day" of the year in the Northern Hemisphere begins on: June 20, 2008 at 7:59 PM EDT
Here's a good site about the Summer Solstice.
As a major celestial event, the Summer Solstice results in the longest day and the shortest night of the year. The Northern Hemisphere celebrates in June, but the people on the Southern half of the earth have their longest summer day in December.
Here is more about it.
In any case - enjoy the day.