Here's some background from Mark Medwin's really excellent review of the set:
As I understand it, the Jazz Workshop was as much a philosophy as an ensemble, necessitating the constant exploration and reconfiguration of whatever music Mingus handed to his musicians. A good portion of the set’s success is in its presentation of live performances in a concentrated chronological space, allowing those reconfigurations to be heard in context. The seven discs span the 16 months from April 1964 through September of 1965. In the booklet notes, Sue Mingus and Brian Priestly discuss the concerns and projects that were coming to a point of culmination, realized or not, during this protracted period. Mingus was still smarting from the strange and disastrous 1962 Town Hall concert, at which he had attempted to record portions of the epic large-ensemble work we now know as Epitaph. His April 1964 return to the venue, the first of the concerts presented here and taking up the first and second discs, finds him in collaboration with what might have been his most sympathetic group, the sextet including Eric Dolphy and Clifford Jordan on winds, the underappreciated Johnny Coles on trumpet, Jaki Byard in the piano chair, and Danny Richmond (the one constant in these performances) on drums. Mingus would take this group to Europe that same month for a series of concerts, and we can hear their Concertgebouw date on the third and fourth discs. Unfortunately, partly due to Dolphy’s death in July, this group would not last to take part in Mingus’s huge triumph at the Monterey Jazz Festival the following September, occupying the fifth disc. A year later, at the same festival and almost to the day, the scene was very different, with scheduling problems causing a much smaller audience and a vastly reduced set time, and we now get to hear the mere half-hour’s worth of frustrated musical vision on the sixth disc. The box set ends resiliently with a May 1965 concert in Minneapolis, during which Mingus denounces major labels, in effect biting the hand that was not feeding him what he’d earned.
There's also a lot of information about the sessions and players at the label site.