An insanely rare musical moment will happen Saturday.

Back in 1987, experimental composer John Cage unveiled Organ2/ASLSP (As Slow as Possible), a piece that when performed on piano lasts anywhere from 20 to 70 minutes. But Cage never actually specified how slow it could be played. A bunch of philosophers said “Challenge accepted!”

These big brains got together in 1997 to debate what Cage meant by “as slow as possible” given that you can hold a note on an organ indefinitely as long as you have power. They then devised a performance that will last for 639 years.

Why 639 years? A special organ was constructed in Germany’s Halberstadt Cathedral in the state of Saxony-Anhalt, the site of the first documented use of a permanent organ back in 1361. This new performance was to begin in 2000, 639 years since that first organ was fired up.

Things didn’t get underway on schedule, though, as it took until 2001 to get the new organ up and running. The official start date was September 5, 2001 (what would have been Cage’s 89th birthday), although the first notation on the score (“Impulse 1”) required a pause lasting until February 5, 2003, a total of 518 days of silence. The first notes were g#′, h′, g#″.

Since then, there have been 14 tone changes (13, if you don’t count that initial pause). “Impulse 15″ will occur this Saturday when the organ will shift from the d#’, a#’, e” that’s been droning since October 13, 2013 (2,527 days, the longest note of the entire performance so far) to g#, e’, which will be held until February 5, 2022 (518 days).

There are many more impulses to go as the score is spread over eight pages. The performance will wrap up on September 5, 2640. Then we’ll get the extended dance remix.

See some picture of the organ here.