At a stadium concert, I and others saw a disk silently hover directly overhead, and then accelerate away at great speed.

Watching a Paul McCartney concert at the Iowa State football stadium, I noticed a flying object slowly approach, thinking at first it was an advertising airplane or a helicopter trying to get a look at the show.  It came directly overhead and hovered completely silently for 3 minutes.  It was a clearly visible disk, 20-foot diameter, with multi-colored lights ringing the underside.  Then it tilted and revealed its top side, which had a solid white light ring around it.  Then it flew away continuing in the direction it had been moving, rapidly accelerating until it was out of sight within a few seconds.  I would estimate the speed to be greater than that of a conventional jet.
 Several people in the seats around me also witnessed this and all believed it to be a UFO.  Some of them borrowed my binoculars for a better look.  Interestingly, immediately afterward, no one was willing to talk about it or acknowledge what they had just seen.  The person I was at the show with also saw it but has been unwilling to discuss it since.
 Footnote: Although I was at a rock concert, I did not drink or take any drugs that night.
Date may be approximate, although the witness may have been able to establish  the exact date of the concert from a ticket stub, check book, etc..  PD