Summer solstice occurs today at 18:24HST. Today the Sun will occupy the most northerly position in the sky of the year. The term solstice comes from the latin terms Sol (the Sun) and sistere (to stand still). On this day the Sun seems to stand still as it stops moving northwards each day and begins move to the south. This is the first day of summer as marked by many cultures in the northern hemisphere. Alternately this is the first day of winter for those living south of the equator.
This year many calendars will mark September 21st as the summer solstice, and so it is for much of the world. Here in Hawaiʻi the solstice actually occurs on the 20th when considering the time zone differences.