Deadlists suggests that there was some question whether or not the Grateful Dead actually played the Mississippi River Festival in Edwardsville, IL on July 8, 1970. Since they played the Fillmore East the next night, the gig may have seemed unlikely. However, the review (above) in the Alton (IL) Evening Telegraph of July 9, 1970 puts that to rest. The Dead played an excellent, well-attended show.
Although the article is primarily a news article, writer Doug Thompson does describe the scene:
The Grateful Dead, a California-based "Acid Rock" group, drew a crowd of 8,500 young persons Wednesday night to the Mississippi River Festival--the largest attendance thus far of the current season.
He goes on to describe that after a late start, the band began with low-key folk and country tunes, and then "livened up the concert with a 23-minute rendition of "Good Lovin'" that included drum solos by two drummers playing simultaneously." The article continues on to describe the scene at some length, but much of the reporters time was spent amongst the crowd or backstage, rather than reviewing the band. Extra special attention is paid to what the young women in the crowd were wearing, which turns out to be not that much, but obviously the news must be reported as it happened.
As near as I can tell, the Mississippi River Festival was at least a week long, possibly longer, and held on a temporary site on the banks of the Mississippi River. There appear to have been temporary pavilions, using large tents. In between talking about miniskirts, the writer alludes to the fact that the Dead were the first rock group to play the Festival that year, and mentions that upcoming acts include composer Henry Mancini (presumably conducting an orchestra) and jazz saxophonist Cannonball Adderley.
As to the Dead's touring schedule, Amalie Rothschild mentioned in her fine book Live At The Fillmore East that the Fillmore East's sound system was so good it was the only venue where the Dead used the house sound instead of their own. Since Rothschild was not only Fillmore East staff herself, but later married the house soundman, she was in a position to know. Considering this, the Dead's touring schedule for the month of July makes sense. After finishing up in Canada, the band played the River Festival, and then their equipment turned and headed to San Francisco, while the band took their guitars and backline on a plane to New York. After a weekend at Fillmore East, the Dead returned home also, and band and sound system were recombined for two shows at the Euphoria in San Rafael on July 14 and 16.
This wasn't unprecedented: for many years it was a mystery how the Dead could have played Kirkwood, MO on May 14, 1970 and the Fillmore East on the following day, but they followed the same pattern. The Dead flew to New York for a gig at Fillmore East, leaving their equipment an extra day to get to Temple University in Philadelphia on May 16.