Today is the Summer Solstice in the Northern Hemisphere. For those below the equatorial line, it is the Winter Solstice. The June Solstice usually takes place between June 20-22. For both the UTC and local time of the solstice, go here.
For those in the Northern Hemisphere, it usually is the longest day of sunlight as the North Pole tilts directly towards the sun. Which translates into more sunlight particularly the further north you live. For those more closer to the North Pole (Alaska, parts of Canada, and Scandinavian countries)the sun literally never sets during this time of year. Of course the reverse is true in the Southern Hemisphere. They get less sunlight on the June Solstice and the closer you are to the Antarctic Circle means less sunlight or total night. For them, it is the Winter Solstice.
The coming of summer is usually a time for celebration in many cultures. Festivals in Northern Europe celebrate summer and the fertility of the Earth. Bonfires are lit and homes are decorated to mark the festival. Many cultures honor the sun in some fashion. Modern day pagans and druids also celebrate the day with their own festivals and many go to Stonehenge in England to witness the first rays of summer.