World's Longest Concert (John Cage)
Organ²/ASLSP (As SLow aS Possible) is a musical piece composed by John Cage and is the subject of the slowest and longest-lasting musical performance yet undertaken.
It was originally written in 1987 for organ and is adapted from the earlier work ASLSP 1985; a typical performance of the piano piece lasts for about 20 to 70 minutes.
In 1985, Cage opted to omit the detail of "exactly how slow the piece should be played".
The current organ performance of the piece at St. Burchardi church in Halberstadt, Germany, began in 2001 and is scheduled to have a duration of 639 years, ending in 2640.
Check out John Cage on wiki
BBC coverage of new note:
Here's a blog that provides some more background on the location of the performance: