More early '71 scheduling stuff:
"The Grateful Dead pulled a last-minute cancellation at Northern Illinois University a few weeks ago."
This was in an article published on May 7, 1971 (1). So depending on the meaning of "a few weeks", it could have been in March, when the GD were in Michigan, Wisconsin, Missouri, canceled in Chicago, and Iowa. It could also have been April, but given the locations I think March seems more likely.
(1) Larry Lujack, "Rock: A DJ's status report," Chicago Tribune, May 7, 1971, p. B11.