The first day of summer is Saturday, June 21.
Set Your Clocks
According to the Farmer’s Almanac, the start of summer in the northern hemisphere is 6:51 a.m. June 21. According to TimeandDate.com, the date of the "June solstice" varies from June 20 to June 22, depending on the year in the Gregorian calendar.
The Farmer’s Almanac states that "the word solstice comes from the Latin words for 'sun' and 'to stop,' due to the fact that the sun appears to stop in the sky.” With the sun high overhead, June 21 will be the longest day of the year.