September 4, 1966 was the Sunday of Labor Day weekend, and at the Fillmore the Grateful Dead were on top — over Quicksilver Messenger Service and Country Joe & the Fish — on the same BG-26 poster (Wes Wilson) that carried the whole weekend (wolfgangs.com BG026; trps.org).

Chickenonaunicycle's own note on this date says "First Time GD Headlines At The Fillmore," and it's tempting to run with that as a milestone. I won't, or not without hedging it hard, because the better sources don't support the clean version. Corry Arnold's work places the Dead's first turn as *sole* Fillmore headliners at the weekend of December 9–11, 1966, and explicitly notes they'd "shared top booking a number of times" before that; a Dead performance list shows them already listed at the Fillmore back on July 14, 1966. So September 4 was an early, prominent top-billing on a shared weekend poster — not a documented "first." If you want the real first-headline date, it's three months later. That's the kind of superlative this project exists to catch.

What's not in dispute is the company. This is the Dead over Quicksilver and Country Joe at the Fillmore on a holiday Sunday, a snapshot of the three bands that, more than any others, *were* the early ballroom scene. The Avalon (FD-24, 13th Floor Elevators) and the Matrix (Blues Project) were both still running their own weekends the same night.