The period of popularity lasted from the late sixties until the early eighties. You will still find them in tropical climates but they are no where near as popular as they once were.
August 21st and 22nd in 1968 were Wednesday and Thursday so I'm confused that the author is calling that the weekend.
9-5 M-F jobs were not the norm for festivalgoers back then, but then again Ticketmaster wasn't selling the tickets for $200 markups, either.