On Sunday, September 11, 1966, Bill Graham threw open the Fillmore Auditorium for the Both/And Benefit (AOR-2.74) — a fundraiser for the Both/And, the jazz club on Divisadero, and a rare night where Graham let jazz and rock share one long bill (event and catalog per coau, corroborated on deadnet and jerrybase). It ran as a single continuous show, roughly 6 p.m. to 2 a.m., and the lineup leaned hard into the jazz side of San Francisco's ear: the Jon Hendricks Trio, Elvin Jones, the Joe Henderson Quartet, Big Mama Thornton, the Denny Zeitlin Trio — set against the young ballroom rock of Jefferson Airplane, The Great Society, and Wildflower. Bill Ham & Co. ran the lights. Ralph Gleason in the San Francisco Chronicle and the Mojo Navigator both reported "a record crowd of 2000 paid $2.50 each" (Gleason and Mojo quoted at deadsources; the same figure echoed by Corry Arnold, per hooterollin).

The night's now-legendary footnote wasn't on the poster at all. The Grateful Dead — unbilled — turned up to close the benefit, playing out the small hours on borrowed equipment (the unbilled closing set per Gleason and Mojo, again via deadsources, and per hooterollin). Nothing about the printed bill promised them; they were a surprise last act rather than a headliner, and that's how the contemporary reviews frame it. The poster itself is a modest, jazz-forward piece — attributed to Hank Lelo, though the attribution is soft enough that it's worth flagging as attributed rather than settled, and it is not a Wes Wilson work despite sitting in the Fillmore's Wilson-era run.

Around this same stretch of that first Airplane summer there's a bit of curated lore worth passing along with both hands: Jack Casady was reportedly sounding out Grace Slick about coming over to the Airplane (per hooterollin, Corry Arnold). Treat it as unconfirmed color — the kind of backstage murmur that gets dated loosely to "around then" — not as a fact pinned to this night. The Great Society sharing the Both/And bill makes the timing tidy, but tidiness isn't corroboration.

A few blocks and a different scale away, the Blues Project were deep in a residency at The Matrix, the club Marty Balin co-founded on Fillmore Street. Their stand ran September 7–15, 1966, which puts September 11 squarely mid-run — a working weeknight-into-weekend club date rather than an event. There's no reliable per-night setlist to quote for the 11th, so the honest move is to place them in the room and leave the particulars where the sources leave them: unrecorded.

Three years on, the same date belonged to Fillmore West. On Thursday, September 11, 1969, Graham opened a four-night run (September 11–14) behind Randy Tuten's BG-191 poster: the Steve Miller Band headlining, with the James Cotton Blues Band and Keef Hartley underneath, and Little Princess #109 on lights (bill, dates, and poster per wolfgangs and famsf). It's a clean, well-documented ballroom bill — Miller's blues-rock at the top, Cotton's harmonica in the middle, Hartley's band opening — and a useful marker of how far the scene had traveled from that jazz-club benefit three years earlier: from a 2,000-head fundraiser where the Dead sneaked on last, to a rock house running numbered posters and house light shows as a matter of course.

(1969: BG-191's Steve Miller / James Cotton / Keef Hartley run opened this night at Fillmore West, its first of four — the September 12–14 shows carry onto those pages. 1967 and 1968: no corroborated San Francisco ballroom event surfaces on this date in the reachable record, so they're left as gaps rather than filled. 1965: no SF ballroom scene yet.)