The mystery surrounding the origins and meaning of the prehistoric standing stone circle of Stonehenge in Wiltshire has invited centuries of speculation, theory and fantasy. One observable fact about this remarkable monument is that the heel stone and the embanked avenue within the circle are aligned to the sun's beams at sunset on the winter solstice and sunrise on the summer solstice. For this reason, the solstices, December 21st and June 21st, are particularly popular times for visitors who want to experience the ancient mystery of the stones. Although the free festivals of the 1970s and 1980s are no longer permitted at Stonehenge, tens of thousands of visitors still gather for the summer solstice annually, and it has become an important event for visitors interested in paganism, folklore and alternative religions. Stonehenge is now recognized as a place of worship for Neo-Pagans and Druids whose right to gather there at the solstices has been acknowledged by the local authorities. Travelers who can't make the solstices might also consider the spring or fall equinoxes, on March 20 and September 22, significant celestial dates in the northern hemisphere calendar, when the hours of day and night are equal.