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This is commonly dated as Jan 29, 1970... after much debate, the prevailing thought is that this is Jan 30, 1970 (after all).
Fleetwood Mac was part of a triple bill listed as: The Flock, Fleetwood Mac, and The Grateful Dead on 1/30/70, and 1/31/70. The Grateful Dead played a bust fund benefit on 2/1/70 at The Warehouse as well (w/ Peter Green sitting in with them). Bear often taped the opening acts that played with the Grateful Dead, but this show seems to be from the night before the Grateful Dead played, as they are not listed as playing on 2/1/70.