@melt_the_tek9 said:

sorry, but Trey sitting in with the Dead is not a "Trey side project." A phish member side project would be their own band. Just as Mike Gordon Band is a side project, when Mike played with The Other Ones, that was not a Mike side project, at least I never would consider it to be.

Booo blog!!!

My explanation may have been a bit off, semantically, but the hint is still perfectly valid: as he did on April 11, 2004 and this past weekend, and as he will do this coming weekend, Trey played a non-Phish show.

Obviously, this hint wasn't exceptionally helpful, but it had been way too long since the contestants were stumped, and I'm a firm believer that the more challenging this game is, the greater the number of people there are who are able to participate, and the more fun it ultimately is. I'd thought of making the hint related to the location (either the state of VT or some reference to the song "Higher Ground" , but I feared that either would give it away to easily, not realizing how stuck most of you would get on this one being from an older show.

And as alluded to by @uctweezer, the precedent of MJMs not necessarily being from an outright Phish show has already been set (see here and here). At least this one featured Phish in its entirety. :-)