On Friday, September 13, 1968, the Avalon Ballroom hung a new Family Dog card — FD-137, "Train Trip," a Wes Wilson design co-credited with Kurt Strausse (both names carried on the poster per rockposters and famsf) — and behind it a bill that leaned hard into the blues. John Mayall headlined, and under him stood Big Mama Mae Thornton and the Hound Dog Band featuring Harmonica George, with Black Pearl opening; the run went September 13–15, so the 13th was the Friday opener (bill and dates per rockposters, wolfgangs, and famsf). This was Mayall at a specific hinge in his career: the *Blues from Laurel Canyon* moment. That album was cut only weeks earlier — August 26–28, 1968 — and wouldn't reach shops until November, so what Avalon-goers actually heard was the touring quartet of the period, Mick Taylor on guitar with Stephen Thompson and Colin Allen, rather than anything you could pin to a tracklist (the recording and release dates are the reliable part; I won't guess the setlist). And there was real blues royalty sharing the boards: Thornton, the voice that first cut "Hound Dog," working her own Hound Dog Band — the poster's spelling, not "House Dog" — with Harmonica George on harp. For one Friday the thirteenth, a young English bandleader documenting Laurel Canyon and an original of the form stood on the same Avalon stage.

Across town the same weekend, Bill Graham was running BG-136 at Fillmore West, and September 13 was its second night. That's the Rick Griffin poster carrying Big Brother and the Holding Company over Santana and Chicago Transit Authority, September 12–14 — a bill whose fuller telling belongs to the September 12 page, so here it's enough to note that on the 13th it was mid-run, three bands deep and already a night old. Between the Avalon's blues summit and the Fillmore's Big Brother weekend, the 13th of September 1968 was one of those San Francisco Fridays where you genuinely had to choose.

A year on, September 13, 1969 found Fillmore West in the middle of BG-191, the Randy Tuten poster for the Steve Miller Band with James Cotton and Keef Hartley. That run stretched September 11–14, which puts the 13th squarely mid-run — night three of four — rather than at either bookend (bill and dates per wolfgangs and rockposters). The same evening, out at the ocean end of things, the Family Dog on the Great Highway had It's A Beautiful Day sharing a bill with The Fourth Way, wrapped in the Garden of Delights light show, September 12–14. I'd treat that one gently: there's no printed poster in the catalog for it — what survives reads more like a date-stamp placeholder than a designed card — so call it a documented booking rather than a poster event, and leave it there (per the date-stamp entry; single-threaded, so hedged).

Rewind to 1967 and the action was smaller and closer to the ground. Steve Miller — billed then as the Steve Miller Blues Band — was working a residency at The Matrix with Peter Walker, September 12–14 (per coau and rockprosopography101). No poster; that's club color, the kind of week-long Matrix stand that doesn't leave a lithograph but tells you where a band was on its way up. Two years before he'd headline a Fillmore weekend of his own, Miller was playing the small room on Fillmore Street.

(1966: no distinct September 13 poster event — the Matrix's Blues Project residency is the color of the week, not a dated card. 1965: earlier still, before there was any of this to poster.)