Stonehenge must be close to the most famous archaeological site on the planet. Built primarily during the Neolithic period (ca. 2000 BC), Stonehenge has been the focus of archaeological study for some 400 years, and no doubt a tourist/ritual/religious destination for many more centuries than that. The main purpose of Stonehenge--and I shudder to use that expression--is under debate, and it probably always will be. But Stonehenge is clearly associated with marking the calendar--specifically, the time of the Summer and Winter Solstice when the stones mark the rise of the sun.
I am informed by a reliable witness that it is possible to wander amongst the Stones both before and after Stonehenge opens and closes to the public throughout the year. Just call the Stone Circle Access Line on 01722 343834 during normal office hours to book a place. See the English Heritage website for public opening times.