The 28th was canceled because Garcia was sick. There are a few guys who are 100% sure that the acid punch show was this Friday night, and when confronted with rock-solid contemporary and convergent evidence, from multiple streams, that 1) the Friday 28th show was canceled and 2) the acid punch show was Saturday 29th, basically reply "fake news". Anyway, that this was canceled is clear from listing in the San Francisco Examiner the next day and from the daybook of Gary Jackson, who was running Winterland shows for Wolfgang at this time. Note, too, that the NRPS played another Peninsula School gig 5/28, w/o Jerry, per the Cryptical Developments blog. DO NOT believe the people who say that the GD and NRPS played Friday night. They did not.
Cxl: "Grateful Dead at Winterland," San Francisco Chronicle, May 29, 1971, p. 32
Cxl: Gary Jackson Archives, 1971 datebook;
Ad: San Francisco Sunday Examiner and Chronicle Datebook, May 23, 1971, p. 7
Mention: "Bay Sounds," Oakland Tribune, May 26, 1971, p. 25