I got this from English Heritage:
The Autumn Equinox occurs at 14.49 on the 22 September 2012, therefore it will be celebrated at Stonehenge at dawn on Saturday 22 September 2012.
Sunrise is at approximately 06.56.
English Heritage is pleased to be offering Managed Open Access for those who wish to celebrate the Autumn Equinox peacefully.
The celebrations will take place at sunrise (approximately 06:56 hrs), however access might restricted/prevented if the ground conditions are poor or if it is felt that access could result in severe damage to the Monument.
Limited facilities are available at Stonehenge for the duration of the access although these facilities will not be available prior to access commencing. Please note there is no car parking available on site.
Visitors will be allowed into the Monument when it is considered sufficiently light to ensure safe access.
The access will be from 06:00 – 06:15 through to 8.00am when visitors will be asked to vacate the site.
And their standard, eminently sensible, conditions of entry:
English Heritage is pleased to be providing Managed Open Access to Stonehenge for the Autumn Equinox. Please help us to create a peaceful occasion by taking personal responsibility and following the Conditions of Entry set out below. We have a duty of care to ensure public safety and to protect Stonehenge and its surrounding monuments. Please read and observe the entry conditions, as this is essential if we are to ensure that future access is sustainable.
English Heritage continues to work closely with the many agencies and people from all sectors of the community and would like to thank them for their help and support.
Please note that the Managed Open Access will cease at 8.00am when the monument field needs to be vacated.
Please observe the following:
These Conditions of Entry are set to ensure enjoyment and public safety for everyone. Contravention of any of these conditions may result in admission being refused or removal from Stonehenge. Drunken, disorderly or antisocial behaviour will not be tolerated.
• THE STONES
Please do not climb on, stand or lean against the stones. This is in the interest of personal safety, the protection of this special site and respect for those attending. As well as putting the stones themselves at risk, climbing on them can damage the delicate lichens that cover them, this includes the stones that are fallen.
• FIRES OR FIREWORKS
Please do not bring or light fires, flaming torches, candles, night-lights, fireworks, Chinese lanterns and other real flames or bring flammable substances or fuel. This applies to the stones and to the land that surrounds them. The nature of this access makes real flame extremely dangerous; it infringes local bylaws and regulations and constitutes a potential fire hazard which is against stringent Health and Safety guidelines that we must observe.
• AMPLIFIED MUSIC
Stonehenge is a prehistoric monument of pre-eminence. It is seen by many as a sacred site and amplified music is not appropriate. No amplified music will be permitted at Stonehenge or in the surrounding landscape. Acoustic instruments will be permitted.
• DOGS OR OTHER ANIMALS
Please do not bring dogs. Dogs, pets or other creatures are not permitted into Stonehenge except registered Guide Dogs. Apart from potentially upsetting wildlife and stock in the area, animal faeces present a health risk to people walking barefoot and children.
• BOTTLES, GLASS OR OTHER OBJECTS
Please note that glass of any type will not be allowed into the monument as many people walk barefoot, and in addition, livestock and wildlife also graze in the area.
WE WOULD LIKE TO WISH YOU A
PEACEFUL AND CELEBRATORY AUTUMN EQUINOX
Please note there is no public car parking available at Stonehenge in the period leading up to and during the Autumn Equinox access.