For the Northern Hemisphere, the 2013 summer solstice will be upon us on Friday. Beginning at 1:04 a.m. EST (5:04 UTC, 10:04 p.m. PST on Thursday) on June 21, the date marks the time when the sun reaches its northernmost point in the sky, yielding the year’s longest stretch of daylight.

The sun can be seen at its highest point straight overhead along the Tropic of Cancer during solstice. However, since the sun in the Northern Hemisphere won’t rise until hours after the solstice, there will be plenty of daylight to enjoy on Friday, up to 16 hours' worth, depending on where you are.

Depending on the calendar year, the summer solstice happens annually in December for the Southern Hemisphere and on June 20 or 21 in the northern half of the world. For science aficionados, the summer solstice happens precisely when the Earth's axial tilt is most inclined toward the sun, at the degree of 23° 26', at its most extreme. In June, the tilt is toward the sun in the Northern Hemisphere, while the second yearly solstice, the winter solstice in December, is away from the sun in the Southern Hemisphere.

But less scientific types, the solstice marks the first day of summer in the Northern Hemisphere, and that means  flip-flops, beach trips and barbecues. In southern England, thousands flock to Stonehenge to see the sun rise from the vantage point of the 4,000-year-old solar monument.

The summer solstice is also a time of celebration for Christians and Pagans. In Christianity, the first day of summer marks the festival of St. John the Baptist, and in Paganism followers celebrate what they call "midsummer" with bonfires and feasts.