Sol + stice derives from a combination of Latin words meaning "sun" + "to stand still." As the days lengthen, the sun rises higher and higher until it seems to stand still in the sky.
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.
Yay! :) Happy first day of summer!