Här är vad High Times har att säga om saken. Ungefär so thc_stoned redan sagt… fast aningens utförligare.

http://www.hightimes.com/ht/lounge/content.php?bid=200&aid=3

…a group known as the “Waldos,” who claim to have originated 420 in San Rafael in 1971. The group began meeting every day at 4:20 PM after school and eventually began to use 420 as a code for marijuana so they could talk about pot in front of parents and teachers.