Summer Solstice 20th/21st June Northern Hemisphere

The longest day has been celebrated for centuries, although to the ancients it was probably not as important as the Winter Solstice.
These days it is mainly the Modern Druids who celebrate this time.

The original Druids were wise men with an aptitude for herblore and learning. But Roman propaganda labelled them as barbarians and as the Romans advanced through Britain the Druids retreated to Anglesey, the small island off the tip of North West Wales.

Eventually the Druids disappeared, but in the 17th century new orders of Druids were established and the culture has grown again... there are several sites across the UK where Druids meet, including Stonehenge in Wiltshire ...several Prehistoric sites seem to have been built to catch the sun on the longest or shortest day and Stonehenge is one of them ...

Stonehenge

... built in Prehistoric times from mostly Welsh stone...the ancient records of the site have long been lost but the Saxon's gave it the name it has today "The Stone Gallows", possibly due to the stone's resembles to the gibbet or gallows that can hang several men at once.
Around circa 1136AD [1136CE] it was stated that the stones marked the graves of 460 noblemen murdered by Saxons, other accounts say they were dancing giants, turned to stone, but the stones are ancient and the people who placed then there were alike to modern man, although their methods are lost in the mists of time, they had the ability and technology to achieve such a wondrous feat that although several millennia have passed, their stones still mystify and attract!

Stongehenge is neither the oldest or the largest henge in Britain, although it seems the most famous. There were, it seems, at least three phases to its building the first the creation of a mound and a ditch, then probably a wooden structure, this was replaced by blue stone from Wales and part of this was replaced by other stones. Several cultures adapted Stonehenge and it was built by many generations, as it took over one millennia to complete.