There is a September 6, 1969 entry for the Family Dog on the Great Highway that this chronicle records with its uncertainty intact, because the primary source records it that way.

The Family Dog listings give the Grateful Dead and Jefferson Airplane for September 6–7, and then immediately qualify it: the show "may have just been the 6th," it "may not be an FD event as unadvertised," and it "may be a replacement for cancelled August 1 event." Three hedges in one entry, from the people who compiled the venue's own history.

So what can honestly be said is this. Something happened at the Great Highway that weekend involving the Dead and the Airplane, it was probably unadvertised, and its date, its promoter and its relationship to a cancelled August booking are all unresolved. No poster is claimed for it. This chronicle would rather print that than a clean sentence it cannot support — the Great Highway's 1969 run is exactly the kind of loosely-documented, date-coded, sometimes-unadvertised operation where a tidy account is usually a wrong one.