The Acropolis might not be able to open late  due to health & safety restrictions, but the nearby Acropolis Museum is not only open until midnight (free admission after 9pm), but also has a free concert in the courtyard outside the building.
Acropolis Museum 
August Full Moon
Wednesday, 21 August, 2013
The Acropolis Museum celebrates August Full Moon on Wednesday 21 August 2013, with famous melodies of the Greek and world repertoire, performed by the historic Army Band of Athens, in the Museum’s entrance courtyard at 9:30 p.m.
The Army Band was founded in 1825 and for many years it represented the only musical authority in Greece comprising of world renowned music conductors, such as Maggel, Kalomiris and Kessaris, as well as talented soloists. Today, the Army Band of Athens supports all Army events and it participates with great success in world festivals of Army Bands abroad. Captain Mike Hasouris will be conducting the orchestra.
On this occasion, the Museum will remain open from 8 a.m. to 12 midnight (free entry for all visitors from 9 p.m. onwards), giving visitors the opportunity to stroll through the galleries and enjoy the view of the Acropolis under the charming moonlight.