On Friday, September 15, 1967, Victor Moscoso's FD-81 — the "Mist Dance" poster — opened its three-night run at the Avalon Ballroom, with The Youngbloods headlining over The Other Half and Mad River, lights by the North American Ibis Alchemical Co. (bill, dates, and the North American Ibis lightshow credit per wolfgangs and rockposters, cross-checked against coau; September 15 is the opening Friday of the Sep 15–17 stand). Moscoso's vibrating complementary color and dissolving lettering were by then the Avalon house signature, and FD-81 is a clean example of the trick that made his posters hard to read on purpose — the eye can't rest on the boundary between figure and ground.

Here is the part worth slowing down for, because it is the kind of thing that trips up anyone reconstructing this weekend from a stack of posters. On that same weekend the Family Dog was running a *second* bill under a *second* poster — not in San Francisco, but in Denver. FD-D2, drawn by George Hunter and Michael Ferguson (both of the Charlatans), advertised Quicksilver Messenger Service, the Charlatans, and Superband at the Family Dog's Denver outpost at 1601 W. Evans Ave., over three nights, September 15–17, 1967 (bill and dates per coau and rockposters — both give a 15–17 run; a dealer listing describing it as a two-night 15–16 stand is the outlier and is not followed here). This is the Family Dog exporting itself to Colorado — Chet Helms's operation planting a flag a thousand miles east of the Avalon while the Avalon itself kept humming. So the trap is real and worth stating plainly: two different Family Dog posters, same weekend, September 1967 — FD-81 by Moscoso in San Francisco, FD-D2 by Hunter and Ferguson in Denver. They share a promoter and a numbering lineage and nothing else. Keep them apart.

The rest of the San Francisco weekend fills in around the anchor. Over at the Fillmore Auditorium, Bill Graham's BG-83 (poster by Jim Blashfield) carried the Electric Flag, Mother Earth, and the LDM Spiritual Band across September 14–16 — which makes the 15th night two of that stand. And a few blocks of the map away, the Straight Theater had Steve Miller Blues Band, Sopwith Camel, and Billy Roberts on the 15th and 16th, with "Hey Joe" on the bill — which, to head off the obvious confusion, is Billy Roberts's own song, the one he wrote, not a separate act sharing the night (Straight Theater bill per coau and rockprosopography101). Roberts turning up in person to play the song everybody by then associated with other people is a small, good detail.

Move the calendar forward and September 15 keeps landing on the Avalon. In 1968 it was FD-137, the Wes Wilson and Kurt Strausse poster, closing out a September 13–15 run with John Mayall, Big Mama Thornton, and Black Pearl — the 15th being the last night of that one (per wolfgangs). Mayall and Big Mama Thornton on the same closing bill is the sort of pairing the Avalon did well, the British blues revival and the woman who cut "Hound Dog" sharing a stage.

The oddest September 15 belongs to 1969, and it isn't a concert at all. That Monday, at the Family Dog on the Great Highway, Stephen Gaskin held the first of what became his "Monday Night Class" — a free, open consciousness lecture, no band, no poster, just Gaskin and whoever wandered in off the beach road (per rockarchaeology101). It's easy to forget that these halls doubled as gathering rooms on their dark nights; the Monday Night Class is the moment the Great Highway building briefly stopped being a rock venue and became something closer to a church, and it grew large enough that it eventually decamped, caravan and all, to found the Farm in Tennessee. Single-day event, one source, so take the "debut" framing as reported rather than proven — but it's the kind of texture that a poster archive alone would never surface.

Further back, the Matrix supplies the earliest color, and it's the thinnest, so hedge accordingly. In 1965, September 15 was a Wednesday, and the club appears to have had Billy Roberts on with Jean Ball — a bill I'd treat as reported rather than nailed down. A year later, in 1966, the 15th was the closing night of a Blues Project residency at the Matrix, the run having gone September 7–15 — the New York band settling into the little Fillmore Street club for a week and change before clearing out.

(1968: John Mayall closes the Avalon on FD-137. 1969: Stephen Gaskin's Monday Night Class debuts on the Great Highway — no music, no poster. 1966: the Blues Project close out their Matrix residency. 1965: a Billy Roberts night at the Matrix, hedged. And 1967 remains the loud one — Moscoso's Mist Dance opening the Avalon while, the same weekend, a second Family Dog poster hangs in a hall in Denver.)