Stonehenge
The summer solstice has probably been celebrated, in some form, for thousands of years at Stonehenge. While Christianity held sway the site was largely ignored.
During the 20th centuary Stonehenge was revived as a place of religious significance, first by neo-druids and then by adherents of neo-pagan and “new age” beliefs.
From 1972 until 1984 Stonehenge was the site of a free festival. I was there in 1980 to witness the celebrations.