During a 1990 Grateful Dead concert, flyers were passed out around the Oakland-Alameda County Coliseum. These flyers recommended smoking marijuana at 4:20.  The purpose of these flyers is unknown, but the Grateful Dead’s connection with marijuana is infamous. Since these flyers explicitly state 4:20 in reference to cannabis, we should consider this concert as the origin of 420 itself. Since these flyers were passed out at a Grateful Dead concert, 420, by proxy, originated from the Grateful Dead.
This position misconstrues the connection of the Grateful Dead with a member of the Waldos, who actually started 420.  Also, this association does not necessarily indicate that the Grateful Dead coined the term because association is not causation. The link between the Grateful Dead and 420 is casual. The fact that it originated it at a Grateful Dead concert does not mean they started it; it means that someone at or around that concert started it.