The summer solstice is the point at which the Earth's northern hemisphere is tilted the furthest toward the sun, usually (but not always) on June 21st, as it is this year. The sun is at the highest point for us, as shown in this analemma. The celebration of the solstice, or Midsummer has taken place since men first noticed the point of the year with the most daylight. In some countries, the pagan celebrations of old were replaced by a feast day for St. John the Baptist on June 24th, but many of the ancient solstice rituals remain.
Wiccans call this day Litha, or sometimes by the Druidic name Alban Heruin, or "the Light of the Shore". To this day, Neo-Druid ceremonies take place at Stonehenge near Salisbury, England.
More solstice celebrations from all over, after the jump.
In Finland, the summer solstice was called Ukon juhla in honor of the god Ukko before the country was Christianized in the 14th century. Now midsummer is known as Juhannus, which means St. John's Day. Helsinki celebrates with a street festival. There is always much drinking and merrymaking.
In Denmark, St. John's Day is called Sankt Hans Aften. It is a lighthearted holiday, a midsummer excuse for a huge bonfire and party.
Midsummers Day, or Sanziene in Romania involves rituals (all in fun) to predict who your future spouse will be. Of course, these rituals are supposed to be performed by those not yet married. In some areas, young women perform the Wicked Fairies Dance for the celebration of Dragaica.
In Alaska, midsummer means sunlight at midnight! This multiple-exposure photograph shows how the sun skips along the horizon during the night near the Arctic Circle. Midsummer is celebrated in different ways all over the state, but understandably, the parties go later in the northermost communities.
The Fremont Arts Council in Seattle holds an annual Summer Solstice Parade and Pageant, billed as "An unparalleled demonstration of free speech, creativity, art and community." Last year's parade included nude bicyclists. Red Kev has pictures. (Warning: nude bicyclists) More parade pictures here.
You'll find more ways people celebrate the summer Midsummer at Wikipedia. Happy Midsummer to you!