Summer Solstice, Midsummer, &c.

Where I reside, at approximately 38° North Latitude, the actual length time between sunrise and sunset is identical (within half a minute) from 15 June to 25 June (and maybe beyond that, I didn’t check). Namely we have about 15 hours 32 minutes between sunrise and sunset – it seems longer because of long morning and evening twilight and often a lingering glow from atmospheric conditions in summer. 
In modern astronomical terms, the solstice is defined in terms of the position of the earth in its orbit around the sun, as measured against the background of the fixed stars.  So in 2017, the solstice occurs at 9:24 PM PDT on 20 June.  
In ancient observational astronomical terms, the solstice is literally the apparent “standing still” of the sun at either end of its travel, over the course of the year, north and south on the ecliptic.  Seeing the sun come up in the same spot for ten or more days is remarkable, compared to the day-to-day changes in equinoctial seasons (more rapidly changing position of sunrise/sunset as well as many minutes per day longer or shorter time of daylight).  So the ancients marked the year with Midsummer celebrations at Summer Solstice, and with Midwinter celebrations (Yuletide) at the Winter Solstice.   
Let me beat my “First Day of Summer” drum.  In my attuned-to-ancient-Celtic brain, the first day of summer was the cross-quarter on Cinco de Mayo, more or less.  First Day of Spring is Groundhog Day (Lunar New Year, more or less, depending on whether you’re a sun kind of person or a moon kind of person). First Day of Winter is actually around Guy Fawkes Day, November 5, plus or minus.  And First Day of Autumn is August 6, Hiroshima Day – or if you prefer, the day on which the racist American President Lyndon Baines Johnson was dragged kicking and screaming and forced into signing the Voting Rights Act of 1965.