The actual date is probably October 16th, though October 9th deserves some consideration.
Here's the operative part of the MN account:
On the 6th of October there was a rally in the Golden Gate Park Panhandle with the Grateful Dead, Big Brother & the Holding Co., and the Wildflower, followed last Sunday by another Panhandle festival, the Artists [sic?] Liberation Front's Free Fair. Bands appearing were the Quicksilver Messenger Service, the Grateful Dead, Country Joe and the Fish. ... After the Fair the Family Dog held its first anniversary dance at Avalon, with Big Brother & the Holding Company, sounding great, the Sir Douglas Quintet, sounding terrible, the Oxford Circle, the 13th Floor Elevators, and a couple of jam groups. Jerry Garcia played one song with the Holding Company, which came off extremely well ...
My first thought was that this was October 9th, not October 16th. Here's why.
In American English (unlike, say, French), we usually reserve a phrase like "last Sunday" for something that was more than a day ago. If Sunday was the day prior, we'd just say "yesterday", with "last Sunday" being 8 days prior. Even out 2-3 days I would refer to "this past Sunday" rather than "last Sunday", though this may be my own idiosyncracy.
Sorry to be pedantic (but I'll continue, anyway). Th context of the quote above could be taken to imply October 9th, for two reasons. First, the linkage of the ALFFF event to the October 6th event with the "followed" formulation implies to me that they were closer in time than ten days. Second is the "last Sunday" formulation. This MN was published on Monday, October 17, 1966, and therefore probably written on 10/16. Even if it was written during the day on 10/17, I find the "last Sunday" formulation unlikely to refer to the previous day.
I was originally leaning toward October 9th, but two things militate strongly against (and/or in favor of 10/16).
First, there is the GD date at the Fillmore Auditorium on October 9th. I don't know what evidence there is for this, but I assume it's strong given how impressively things tend to be nailed down for the main ballroom gigs. The logistics also seem to militate against all of these things having happened on the same day. Hard to imagine they could do the Golden Gate Park thing, the Fillmore thing, and the Avalon thing in the same day, unless this was a morning-afternoon-night arrangement. The context given in MN makes this less likely ... in the quote's ellipsis is reference to the fact that it was a warm afternoon. I take all of this as pretty strong against 10/9.
Second, Chicken on a Unicycle shows Big Brother at the Avalon on 10/15-16, but not on 10/9. Having recently spent a lot of time with Ross and Corry's careful research, I have great faith that all of this is correct.
In short, despite the fact that I find the construction of the account to be strange, I think I have to go with 10/16. I'll post shortly about the next Jerry-on-the-side (JOTS, interchangeable with GOTS when I am in a last-name mood) thing from this same night.